CDAW Data Center
Planetary Magnetospheres Branch (Code 695)
Laboratory for Extraterrestrial Physics
NASA / Goddard Space Flight Center
Greenbelt Maryland USA


Publications

ADS listing
N. Gopalswamy: Refereed Journals, Proceedings, Meeting Abstracts
S. Yashiro: Refereed Journals, Proceedings, Meeting Abstracts
H. Xie: Refereed Journals, Proceedings, Meeting Abstracts
S. Akiyama: Refereed Journals, Proceedings, Meeting Abstracts
P. Mäkelä: Refereed Journals, Proceedings, Meeting Abstracts
N. Thakur: Refereed Journals, Proceedings, Meeting Abstracts
C. Kay: Refereed Journals, Proceedings, Meeting Abstracts

Refereed Papers Making Use of CME Catalog Data


2016

Source Regions of the Type II Radio Burst Observed During a CME-CME Interaction on 2013 May 22

P. Mäkelä, N. Gopalswany, M. J. Reiner, S. Akiyama and V. Krupar
accepted for publication in The Astrophysical Journal on June 20, 2016.

Abstract

We report on our study of radio source regions during the type II radio burst on 2013 May 22 based on direction finding (DF) analysis of the Wind/WAVES and STEREO/WAVES (SWAVES) radio observations at decameter-hectometric (DH) wavelengths. The type II emission showed an enhancement that coincided with interaction of two coronal mass ejections (CMEs) launched in sequence along closely spaced trajectories. The triangulation of the SWAVES source directions posited the ecliptic projections of the radio sources near the line connecting the Sun and the STEREO-A spacecraft. The WAVES and SWAVES source directions revealed shifts in the latitude of the radio source indicating that the spatial location of the dominant source of the type II emission varies during the CME-CME interaction. The WAVES source directions close to 1 MHz frequencies matched the location of the leading edge of the primary CME seen in the images of the LASCO/C3 coronagraph. This correspondence of spatial locations at both wavelengths confirms that the CME-CME interaction region is the source of the type II enhancement. Comparison of radio and white-light observations also showed that at lower frequencies scattering significantly affects radio wave propagation.

A preprint of this paper can be downloaded from arXiv.


The Radial Speed - Expansion Speed Relation for Earth-Directed CMEs

P. Mäkelä, N. Gopalswany, and S. Yashiro
accepted for publication in Space Weather on May 4, 2016.

Abstract

Earth-directed coronal mass ejections (CMEs) are the main drivers of major geomagnetic storms. Therefore, a good estimate of the dis- turbance arrival time at Earth is required for space weather predictions. The STEREO and SOHO spacecraft were viewing the Sun in near-quadrature during January 2010- September 2012, providing a unique opportunity to study the radial speed (Vrad) - expansion speed (Vexp) relationship of Earth- directed CMEs. This relationship is useful in estimating the Vrad of Earth- directed CMEs, when they are observed from Earth-view only. We selected 19 Earth-directed CMEs observed by the LASCO/C3 coronagraph on SOHO and the SECCHI/COR2 coronagraph on STEREO during January 2010-September 2012. We found that of the three tested geometric CME models the full ice- cream cone model of the CME describes best the Vrad -Vexp relationship, as suggested by earlier investigations. We also tested the prediction accuracy of the empirical shock arrival (ESA) model proposed by Gopalswamy et al. [2005a], while estimating the CME propagation speeds from the CME ex- pansion speeds. If we use STEREO observations to estimate the CME width required to calculate the Vrad from the Vexp measurements, the mean abso- lute error (MAE) of the shock arrival times of the ESA model is 8.4 hours. If the LASCO measurements are used to estimate the CME width, the MAE still remains below 17 hours. Therefore by using the simple Vrad -Vexp rela- tionship to estimate the Vrad of the Earth-directed CMEs, the ESA model is able to predict the shock arrival times with accuracy comparable to most other more complex models. Earth-directed coronal mass ejections (CMEs) are the main

A preprint of this paper can be downloaded as a pdf file.


On the Directivity of Low-Frequency Type IV Radio Bursts

Nat Gopalswamy, Sachiko Akiyama, Pertti Mäkelä, Seiji Yashiro, and Iver H. Cairns
contributed paper to b presented at the URSI Asia-Pacific Radio Science Conference in Seoul, August 21-25, 2016

Abstract

An intense type IV radio burst was observed by the STEREO Behind (STB) spacecraft located about 144 degres behind Earth. The burst was associated with a large solar eruption that occurred on the backside of the Sun (N05E151) close to the disk center in the STB view. The eruption was also observed by the STEREO Ahead (STA) spacecraft (located at 149 degrees ahead of Earth) as an eruption close to the west limb (N05W60) in that view. The type IV burst was complete in STB observations in that the envelope reached the lowest frequency and then receded to higher frequencies. The burst was partial viewed from STA, revealing only the edge coming down to the lowest frequency. The type IV burst was not observed at all near Earth because the source was 61 degrees behind the east limb. The eruption was associated with a low-frequency type II burst observed in all three views, although it was not very intense. Solar energetic particles were also observed at both STEREOs and at SOHO, suggesting that the shock was much extended, consistent with the very high speed of the CME (about 2048 km/s). These observations suggest that the type IV emission is directed along a narrow cone above the flare site. We confirm this result statistically using the type IV bursts of solar cycle 23.

A preprint of this paper can be downloaded from arXiv.


Solar Activity Studies using Microwave Imaging Observations

Nat Gopalswamy
invited paper to the URSI Asia-Pacific Radio Science Conference in Seoul, August 21-25, 2016

Abstract

We report on the status of solar cycle 24 based on polar prominence eruptions (PEs) and microwave brightness enhancement (MBE) information obtained by the Nobeyama radioheliograph. The north polar region of the Sun had near-zero field strength for more than three years (2012 to 2015) and ended only in September 2015 as indicated by the presence of polar PEs and the lack of MBE. The zero-polar-field condition in the south started only around 2013, but it ended by June 2014. Thus the asymmetry in the times of polarity reversal switched between cycle 23 and 24. The polar MBE is a good proxy for the polar magnetic field strength as indicated by the high degree of correlation between the two. The cross-correlation between the high- and low-latitude MBEs is significant for a lag of ~5.5 to 7.3 years, suggesting that the polar field of one cycle indicates the sunspot number of the next cycle in agreement with the Babcock-Leighton mechanism of solar cycles. The extended period of near-zero field in the north-polar region should result in a weak and delayed sunspot activity in the northern hemisphere in cycle 25.

A preprint of this paper can be downloaded from arXiv.


Low-Frequency Radio Bursts and Space Weather

Nat Gopalswamy
invited paper to the URSI Asia-Pacific Radio Science Conference in Seoul, August 21-25, 2016

Abstract

Low-frequency radio phenomena are due to the presence of nonthermal electrons in the interplanetary (IP) medium. Understanding these phenomena is important in characterizing the space environment near Earth and other destinations in the solar system. Substantial progress has been made in the past two decades, because of the continuous and uniform data sets available from space-based radio and white-light instrumentation. This paper highlights some recent results obtained on IP radio phenomena. In particular, the source of type IV radio bursts, the behavior of type III storms, shock propagation in the IP medium, and the solar-cycle variation of type II radio bursts are considered. All these phenomena are closely related to solar eruptions and active region evolution. The results presented were obtained by combining data from the Wind and SOHO missions.

A preprint of this paper can be downloaded from arXiv.


Unusual Polar Conditions in Solar Cycle 24 and their Implications for Cycle 25

Nat Gopalswamy, Seiji Yashiro, and Sachiko Akiyama
Accepted for publication in ApJ Lett. on May 6, 2016

Abstract

We report on the prolonged solar-maximum conditions until late 2015 at the north-polar region of the Sun indicated by the occurrence of high-latitude prominence eruptions and microwave brightness temperature close to the quiet Sun level. These two aspects of solar activity indicate that the polarity reversal was completed by mid-2014 in the south and late 2015 in the north. . The microwave brightness in the south-polar region has increased to a level exceeding the level of cycle 23/24 minimum, but just started to increase in the north. The north-south asymmetry in the polarity reversal has switched from that in cycle 23. These observations lead us to the hypothesis that the onset of cycle 25 in the northern hemisphere is likely to be delayed with respect to that in the southern hemisphere. We find that the unusual condition in the north is a direct consequence of the arrival of poleward surges of opposite polarity from the active region belt. We also find that multiple rush-to-the-pole episodes were indicated by the prominence eruption locations that lined up at the boundary between opposite polarity surges. The high-latitude prominence eruptions occurred in the boundary between the incumbent polar flux and the insurgent flux of opposite polarity.

A preprint of this paper can be downloaded from arXiv.


Two Exceptions in the Large SEP Events of Solar Cycles 23 and 24

N. Thakur, N. Gopalswamy, P. Mäkelä, S. Akiyama, S. Yashiro, and H. Xie
Accepted for Solar Physics, March, 2016

Abstract

We discuss our findings from a survey of all large solar energetic particle (SEP) events of Solar Cycles 23 and 24, i.e. the SEP events where the intensity of >10 MeV protons observed by GOES was >10 pfu. In our previous work (Gopalswamy et al., 2014 Geophys. Res. Lett. 41, 2673) we suggested that ground level enhancements (GLEs) in Cycles 23 and 24 also produce an intensity increase in the GOES >700 MeV proton channel. Our survey, now extended to include all large SEP events of Cycle 23, confirms this to be true for all but two events: i) the GLE of 6 May 1998 (GLE57) for which GOES did not observe enhancement in >700 MeV protons intensities and ii) a high-energy SEP event of 8 November 2000, for which GOES observed >700 MeV protons but no GLE was recorded. Here we discuss these two exceptions. We compare GLE57 with other small GLEs, and the 8 November 2000 SEP event with those that showed similar intensity increases in the GOES >700 MeV protons but produced GLEs. We find that because GOES >700 MeV proton intensity enhancements are typically small for small GLEs, they are difficult to discern near solar minima due to higher background. Our results also support that GLEs are generally observed when shocks of the associated coronal mass ejections (CMEs) form at heights 1.2-1.93 solar radii [RSUN] and when the solar particle release occurs between 2-6 RSUN. Our secondary findings support the view that the nose region of the CME-shock may be accelerating the first-arriving GLE particles and the observation of a GLE is also dependent on the latitudinal connectivity of the observer to the CME-shock nose. We conclude that the GOES >700 MeV proton channel can be used as an indicator of GLEs excluding some rare exceptions, such as those discussed here.

A preprint of this paper can be downloaded as a pdf file.


History and Development of Coronal Mass Ejections as a Key Player in Solar Terrestrial Relationship

Nat Gopalswamy
Accepted for Publication in Geoscience Letters, February 10, 2016

Abstract

oronal mass ejections (CMEs) are relatively a recently-discovered phenomenon, in 1971, some fifteen years into the Space Era. It took another two decades to realize that CMEs are the most important players in solar terrestrial relationship as the root cause of severe weather in Earth's space environment. CMEs are now counted among the major natural hazards because they cause large solar energetic particle (SEP) events and major geomagnetic storms, both of which pose danger to humans and their technology in space and ground. Geomagnetic storms discovered in the 1700s, solar flares discovered in the 1800s, and SEP events discovered in the1900s are all now found to be closely related to CMEs via various physical processes occurring at various locations in and around CMEs, when they interact with the ambient medium. This article identifies a number of key developments that preceded the discovery of white-light CMEs suggesting that CMEs were waiting to be discovered. The last two decades witnessed an explosion of CME research following the launch of the Solar and Heliospheric Observatory mission in 1995, resulting in the establishment of a full picture of CMEs.

A preprint of this paper can be downloaded as a pdf file.


2015

Advancing the understanding of the Sun-Earth interaction - the Climate and Weather of the Sun-Earth System (CAWSES) II program

Toshitada Tsuda, Marianna Shepherd and Nat Gopalswamy
Progress in Earth and Planetary Science 2015, 2, 28

Abstract

The Scientific Committee on Solar-Terrestrial Physics (SCOSTEP) of the International Council for Science (ICSU) implemented an international collaborative program called Climate and Weather of the Sun-Earth System (CAWSES), which was active from 2004 to 2008; this was followed by the CAWSES II program during the period of 2009-2013. The CAWSES program was aimed at improving the understanding of the coupled solar-terrestrial system, with special emphasis placed on the short-term (weather) and long-term (climate) variability of solar activities and their effects on and responses of Geospace and Earth's environment. Following the successful implementation of CAWSES, the CAWSES II program pursued four fundamental questions addressing the way in which the coupled Sun-Earth system operates over time scales ranging from minutes to millennia, namely, (1) What are the solar influences on the Earth's climate? (2) How will Geospace respond to an altered climate? (3) How does short-term solar variability affect the Geospace environment? and (4) What is the Geospace response to variable inputs from the lower atmosphere? In addition to these four major tasks, the SCOSTEP and CAWSES promoted E-science and informatics activities including the creation of scientific databases and their effective utilization in solar-terrestrial physics research. Capacity building activities were also enhanced during CAWSES II, and this represented an important contribution of SCOSTEP to the world's solar-terrestrial physics community. This introductory paper provides an overview of CAWSES II activities and serves as a preface to the dedicated review papers summarizing the achievements of the program's four task groups (TGs) and the E-science component.

A preprint of this paper can be downloaded as a pdf file.


CMEs during the Two Activity Peaks in Cycle 24 and their Space Weather Consequences

Nat Gopalswamy, P. Mäkelä, Sachiko Akiyama, Seiji Yashiro, and N. Thakur
Accepted for publication in Sun and Geosphere, September 12, 2015

Abstract

We report on a comparison between space weather events that occurred around the two peaks in the sunspot number (SSN) during solar cycle 24. The two SSN peaks occurred in the years 2012 and 2014. Even though SSN was larger during the second peak, we find that there were more space weather events during the first peak. The space weather events we considered are large solar energetic particle (SEP) events and major geomagnetic storms associated with coronal mass ejections (CMEs). We also considered interplanetary type II radio bursts, which are indicative of energetic CMEs driving shocks. When we compared the CME properties between the two SSN peaks, we find that more energetic CMEs occurred during the 2012 peak. In particular, we find that CMEs accompanying IP type II bursts had an average speed of 1543 km/s during the 2012 peak compared to 1201 km/s during the 2014 peak. This result is consistent with the reduction in the average speed of the general population of CMEs during the second peak. All SEP events were associated with the interplanetary type II bursts, which are better than halo CMEs as indicators of space weather. The comparison between the two peaks also revealed the discordant behavior between the CME rate and SSN was more pronounced during the second peak. None of the 14 disk-center halo CMEs was associated with a major storm in 2014. The lone major storm in 2014 was due to the intensification of the (southward) magnetic field in the associated magnetic cloud by a shock that caught up and propagated into the magnetic cloud.

A preprint of this paper can be downloaded as a pdf file.


The Mild Space Weather in Solar Cycle 24

Nat Gopalswamy, Sachiko Akiyama, Seiji Yashiro, Hong Xie, P. Mäkelä, and Grzegorz Michalek
the Proc. 14th International Ionospheric Effects Symposium on 'Bridging the gap between applications and research involving ionospheric and space weather disciplines' May 12-14, 2015, Alexandria, VA

Abstract

The space weather is extremely mild during solar cycle 24: the number of major geomagnetic storms and high-energy solar energetic particle events are at the lowest since the dawn of the space age. Solar wind measurements at 1 AU using Wind and ACE instruments have shown that there is a significant drop in the density, magnetic field, total pressure, and Alfven speed in the inner heliosphere as a result of the low solar activity. The drop in large space weather events is disproportionately high because the number of energetic coronal mass ejections that cause these events has not decreased significantly. For example, the rate of halo CMEs, which is a good indicator of energetic CMEs, is similar to that in cycle 23, even though the sunspot number has declined by about 40\%. The mild space weather seems to be a consequence of the anomalous expansion of CMEs due to the low ambient pressure in the heliosphere. The anomalous expansion results in the dilution of the magnetic contents of CMEs, so the geomagnetic storms are generally weak. CME driven shocks propagating through the weak heliospheric field are less efficient in accelerating energetic particles, so the particles do not attain high energies. Finally, we would like to point out that extreme events such as the 2012 July 23 CMEs that occurred on the backside of the Sun and did not affect Earth except for a small proton event.

A preprint of this paper can be downloaded from arXiv.


High-energy solar particle events in cycle 24

N. Gopalswamy, P. Mäkelä, S. Yashiro, H. Xie, S. Akiyama, and N. Thakur
Accepted for publication in Journal of Physics: Conference Series, July 21, 2015

Abstract

The Sun is already in the declining phase of cycle 24, but the paucity of high-energy solar energetic particle (SEP) events continues with only two ground level enhancement (GLE) events as of March 31, 2015. In an attempt to understand this, we considered all the large SEP events of cycle 24 that occurred until the end of 2014. We compared the properties of the associated CMEs with those in cycle 23. We found that the CME speeds in the sky plane were similar, but almost all those cycle-24 CMEs were halos. A significant fraction of (16%) of the frontside SEP events were associated with eruptive prominence events. CMEs associated with filament eruption events accelerate slowly and attain peak speeds beyond the typical GLE release heights. When we considered only western hemispheric events that had good connectivity to the CME nose, there were only 8 events that could be considered as GLE candidates. One turned out to be the first GLE event of cycle 24 (2012 May 17). In two events, the CMEs were very fast (>2000 km/s) but they were launched into a tenuous medium (high Alfven speed). In the remaining five events, the speeds were well below the typical GLE CME speed (~2000 km/s). Furthermore, the CMEs attained their peak speeds beyond the typical heights where GLE particles are released. We conclude that several factors contribute to the low rate of high-energy SEP events in cycle 24: (i) reduced efficiency of shock acceleration (weak heliospheric magnetic field), (ii) poor latitudinal and longitudinal connectivity), and (iii) variation in local ambient conditions (e.g., high Alfven speed).

A preprint of this paper can be downloaded as a pdf file.


Kinematic and Energetic Properties of the 2012 March 12 Polar Coronal Mass Ejection

N. Gopalswamy, S. Yashiro, and S. Akiyama
Accepted for publication in the Astrophysical Journal, July 14, 2015

Abstract

We report on the energetics of the 2012 March 12 polar coronal mass ejection (CME) originating from a southern latitude of ~60°. The polar CME is similar to low-latitude CMEs in almost all respects: three-part morphology, post eruption arcade (PEA), CME and filament kinematics, CME mass and kinetic energy, and the relative thermal energy content of the PEA. From polarized brightness images, we estimate the CME mass, which is close to the average mass of low-latitude CMEs. The CME kinetic energy (3.3x1030 erg) is also typical of the general population of CMEs. From photospheric magnetograms, we estimate the free energy (1.8x1031 erg) in the polar crown source region, which we find is sufficient to power the CME and the PEA. About 19% of the free energy went into the CME kinetic energy. We compute the thermal energy content of the PEA (2.3x1029 erg) and find it to be a small fraction (6.8%) of the CME kinetic energy. This fraction is remarkably similar to that in active region CMEs associated with major flares. We also show that the 2012 March 12 is one among scores of polar CMEs observed during the maximum phase of cycle 24. The cycle 24 polar crown prominence eruptions have the same rate of association with CMEs as those from low-latitudes. This investigation supports the view that all CMEs are magnetically propelled from closed field regions, irrespective of their location on the Sun (polar crown filament regions, quiescent filament regions or active regions).

A preprint of this paper can be downloaded as a pdf file.


Short-term Variability of the Sun -Earth System: An Overview of Progress Made during the CAWSES-II Period

Nat Gopalswamy, Bruce Tsurutani, and Yihua Yan
Progress in Earth and Planetary Science, accepted on April 13, 2015

Abstract

This paper presents an overview of results obtained during the CAWSES-II period on the short-term variability of the Sun and how it affects the near-Earth space environment. CAWSES-II was planned to examine the behavior of the solar-terrestrial system as the solar activity climbed to its maximum phase in solar cycle 24. After a deep minimum following cycle 23, the Sun climbed to a very weak maximum in terms of the sunspot number in cycle 24 (MiniMax24), so many of the results presented here refer to this weak activity in comparison with cycle 23. The short-term variability that has immediate consequence to Earth and geospace manifests as solar eruptions from closed-field regions and high-speed streams from coronal holes. Both electromagnetic (flares) and mass emissions (coronal mass ejections - CMEs) are involved in solar eruptions, while coronal holes result in high-speed streams that collide with slow wind forming the so-called corotating interaction regions (CIRs). Fast CMEs affect Earth via leading shocks accelerating energetic particles and creating large geomagnetic storms. CIRs and their trailing high speed streams (HSSs), on the other hand, are responsible for recurrent small geomagnetic storms and extended (days) of auroral zone activity, respectively. The latter lead to the acceleration of relativistic magnetospheric “killer” electrons. One of the major consequences of the weak solar activity is the altered physical state of the heliosphere that has serious implications for the shock-driving and storm-causing properties of CMEs. Finally, a discussion is presented on extreme space weather events prompted by the 2012 July 23 super storm event that occurred on the backside of the Sun. Many of these studies were enabled by the simultaneous availability of remote-sensing and in-situ observations from multiple vantage points with respect to the Sun-Earth line.

A preprint of this paper can be downloaded as a pdf file.


The Peculiar Behavior of Halo Coronal Mass Ejections in Solar Cycle 24

N. Gopalswamy, H. Xie, S. Akiyama, P. Mäkelä, S. Yashiro, and G. Michalek
Accepted for

Abstract publication in the Astrophysical Journal, April 7, 2015

We report on a remarkable finding that the halo coronal mass ejections (CMEs) in cycle 24 are more abundant than in cycle 23, although the sunspot number in cycle 24 has dropped by ∼40%. We also find that the distribution of halo-CME source locations is different in cycle 24: the longitude distribution of halos is much flatter with the number of halos originating at central meridian distance ≥60° twice as large as that in cycle 23. On the other hand, the average speed and the associated soft X-ray flare size are the same in the two cycles, suggesting that the ambient medium into which the CMEs are ejected is significantly different. We suggest that both the higher abundance and larger central meridian longitudes of halo CMEs can be explained as a consequence of the diminished total pressure in the heliosphere in cycle 24 (Gopalswamy et al. 2014). The reduced total pressure allows CMEs expand more than usual making them appear as halos.

A preprint of this paper can be downloaded as a pdf file.


Estimating the Height of CMEs Associated with a Major SEP Event at the Onset of the Metric Type II Radio Burst during Solar Cycles 23 and 24

P. Mäkelä, N. Gopalswamy, S. Akiyama, H. Xie, and S. Yashiro
Accepted for publication in the Astrophysical Journal, April 7, 2015

Abstract

We studied the coronal mass ejection (CME) height at the onset of 59 metric type II radio bursts associated with major solar energetic particle (SEP) events, excluding ground level enhancements (GLEs), during solar cycles 23 and 24. We calculated CME heights using a simple flare-onset method used by Gopalswamy et al. (2012b) to estimate CME heights at the metric type II onset for cycle-23 GLEs. We found the mean CME height for non-GLE events (1.72 R◎ ) to be ∼12% greater than that (1.53 R◎ ) for cycle-23 GLEs. The difference could be caused by more impulsive acceleration of the GLE-associated CMEs. For cycle-24 non-GLE events, we compared the CME heights obtained using the flare-onset method and the 3-D spherical-shock fitting method and found the correlation to be good (CC=0.68). We found the mean CME height for cycle 23 non-GLE events (1.79 R◎ ) to be greater than for cycle 24 non-GLE events (1.58 R◎ ), but statistical tests do not definitely reject the possibility of coincidence. We suggest that the lower formation height of the shocks during cycle 24 indicates a change in the Alfvén speed profile because solar magnetic fields are weaker and e plasma density levels are closer to the surface than usual during cycle 24. We also found that complex type III bursts showing diminution of type III emission in the 7-14 MHz frequency range are more likely associated with events with the CME height at the type II onset above 2 R◎ , supporting suggestions that the CME/shock structure causes the feature.

A preprint of this paper can be downloaded as a pdf file.


Large Solar Energetic Particle Events Associated with Filament Eruptions Outside of Active Regions

N. Gopalswamy, P. Mäkelä, S. Akiyama, S. Yashiro, H. Xie, N. Thakur, and S. W. Kahler
Accepted for publication in the Astrophysical Journal, March 31, 2015

Abstract

We report on four large filament eruptions (FEs) from solar cycles 23 and 24 that were associated with large solar energetic particle (SEP) events and interplanetary type II radio bursts. The post-eruption arcades corresponded to mostly C-class soft X-ray enhancements, but an M1.0 flare was associated with one event. However, the associated coronal mass ejections (CMEs) were fast (speeds about 1000 km/s) and appeared as halo CMEs in the coronagraph field of view. The interplanetary type II radio bursts occurred over a wide wavelength range indicating the existence of strong shocks throughout the inner heliosphere. No metric type II bursts were present in three events, indicating that the shocks formed beyond 2 to 3 Rs. In one case, there was a metric type II burst with low starting frequency indicating a shock formation height of about 2 Rs. The FE-associated SEP events did have softer spectra (spectral index greater than 4) in the 10 to 100 MeV range, but there were other low-intensity SEP events with spectral indices ≥ 4. Some of these events are likely FE-SEP events, but were not classified so in the literature because they occurred close to active regions. Some were definitely associated with large active region flares, but the shock formation height was large. We definitely find a diminished role for flares and complex type III burst durations in these large SEP events. Fast CMEs and shock formation at larger distances from the Sun seem to be the primary characteristics of the FE-associated SEP events.

A preprint of this paper can be downloaded from arXiv.


2014

The dynamics of eruptive prominences

N. Gopalswamy
In "Solar Prominences", edited by J.-C. Vial & O. Engvold, Springer, in press (2014)

Abstract

This chapter discusses the dynamical properties of eruptive prominences in relation to coronal mass ejections (CMEs). The fact that eruptive prominences are a part of CMEs is emphasized in terms of their physical association and kinematics. The continued propagation of prominence material into the heliosphere is illustrated using in-situ observations. The solar-cycle variation of eruptive prominence locations is discussed with a particular emphasis on the rush-to-the-pole (RTTP) phenomenon. One of the consequences of the RTTP phenomenon is polar CMEs, which are shown to be similar to the low-latitude CMEs. This similarity is important because it provides important clues to the mechanism by which CMEs erupt. The nonradial motion of CMEs is discussed, including the deflection by coronal holes that have important space weather consequences. Finally, the implications of the presented observations for the modeling CME modeling are outlined.

A preprint of this paper can be downloaded as a pdf file.


Major Solar Eruptions and High Energy Particle Events during Solar Cycle 24

N. Gopalswamy, H. Xie, S. Akiyama, P. Mäkelä, and S. Yashiro
Earth, Planets and Space, Volume 66, article id. 104, 2014

Abstract

We report on a study of all major solar eruptions that occurred on the frontside of the Sun during the rise to peak phase of cycle 24 (first 62 months) in order to understand the key factors affecting the occurrence of large solar energetic particle events (SEPs) and the ground levels enhancement (GLE) events. The eruptions involve major flares with soft X-ray peak flux ≥ 5.0 x10-5 Wm-2 (i.e., flare size ≥M5.0) and accompanying coronal mass ejections (CMEs). The selection criterion was based on the fact that the only front-side GLE in cycle 24 (GLE 71) had a flare size of M5.1.Only ∼37% of the major eruptions from the western hemisphere resulted in large SEP events.Almost the same number of large SEP events was produced in weaker eruptions (flare size <M5.0), suggesting that the soft X-ray flare is not a good indicator of SEP or GLE events. On the other hand, the CME speed is a better indicator of SEP and GLE events because it is consistently high supporting the shock acceleration mechanism for SEPs and GLEs.We found the CME speed, magnetic connectivity to Earth, and ambient conditions as the main factors that contribute to the lack of high energy particle events during cycle 24. Several eruptions poorly connected to Earth (eastern-hemisphere or behind-the-west-limb events) resulted in very large SEP events detected by the STEREO spacecraft.Some very fast CMEs, likely to have accelerated particles to GeV energies, did not result in a GLE event because of poor latitudinal connectivity. The stringent latitudinal requirement suggests that the highest energy particles are likely accelerated in the nose part of shocks. There were also well-connected fast CMEs, which did not seem to have accelerated high energy particles due to possible unfavorable ambient conditions (high Alfven speed, overall reduction in acceleration efficiency in cycle 24).

A preprint of this paper can be downloaded as a pdf file.


Homologous flare-CME events and their metric type II radio burst association

S. Yashiro, N. Gopalswamy, P. Mäkelä, S. Akiyama, W. Uddin, A.K. Srivastava, N.C. Joshi, R. Chandra, P.K. Manoharan, K. Mahalakshmi, V.C. Dwivedi, R. Jain, A.K. Awasthi, N.V. Nitta, M. J. Aschwanden, and D. P. Choudhary
Advances in Space Research, Volume 54, Issue 9, p. 1941-1948, 2014

Abstract

Active region NOAA 11158 produced many flares during its disk passage. At least two of these flares can be considered as homologous: the C6.6 flare at 06:51 UT and C9.4 flare at 12:41 UT on February 14, 2011. Both flares occurred at the same location (eastern edge of the active region) and have a similar decay of the GOES soft X-ray light curve. The associated coronal mass ejections (CMEs) were slow (334 km/s and 337 km/s) and of similar apparent widths (43° and 44°), but they had different radio signatures. The second event was associated with a metric type II burst while the first one was not. The COR1 coronagraphs on board the STEREO spacecraft clearly show that the second CME propagated into the preceding CME that occurred 50 minutes before. These observations suggest that CME-CME interaction might be a key process in exciting the type II radio emission by slow CMEs.

A preprint of this paper can be downloaded as a pdf file.


Ground Level Enhancement in the 2014 January 6 Solar Energetic Particle Event

N. Thakur, N. Gopalswamy, H. Xie, P. Mäkelä, S. Yashiro, S. Akiyama, and J. M. Davila
The Astrophysical Journal Letters, Vol. 790, article id. L13, 2014

Abstract

We present a study of the 2014 January 6 solar energetic particle (SEP) event, which produced a small ground level enhancement (GLE), making it the second GLE of this unusual solar cycle 24. This event was primarily observed by the South Pole neutron monitors (increase of ~2.5%) whereas a few other neutron monitors recorded smaller increases. The associated coronal mass ejection (CME) originated behind the western limb and had the speed of 1960 km/s. The height of the CME at the start of the associated metric type II radio burst, which indicates the formation of a strong shock, was measured to be 1.61 Rs using a direct image from STEREO-A/EUVI. The CME height at the time of GLE particle release (determined using the South Pole neutron monitor data) was directly measured as 2.96 Rs, from the STEREO-A/COR1 white-light observations. These CME heights are consistent with those obtained for the GLE71, the only other GLE of the current cycle as well as cycle-23 GLEs derived using back-extrapolation. GLE72 is of special interest because it is one of the only two GLEs of cycle 24, one of the two behind-the-limb GLEs and one of the two smallest GLEs of cycles 23 and 24.

A preprint of this paper can be downloaded as a pdf file.


Anomalous expansion of coronal mass ejections during solar cycle 24 and its space weather implications

N. Gopalswamy, S. Akiyama, S. Yashiro, H. Xie, P. Mäkelä, and G. Michalek
Geophys. Res. Lett. Vol. 41, Issue 8, pp.2673-2680. 2014

Abstract

The familiar correlation between the speed and angular width of coronal mass ejections (CMEs) is also found in solar cycle 24, but the regression line has a larger slope: for a given CME speed, cycle 24 CMEs are significantly wider than those in cycle 23. The slope change indicates a significant change in the physical state of the heliosphere, due to the weak solar activity. The total pressure in the heliosphere (magnetic + plasma) is reduced by ~40%, which leads to the anomalous expansion of CMEs explaining the increased slope. The excess CME expansion contributes to the diminished effectiveness of CMEs in producing magnetic storms during cycle 24, both because the magnetic content of the CMEs is diluted and also because of the weaker ambient fields. The reduced magnetic field in the heliosphere may contribute to the lack of solar energetic particles accelerated to very high energies during this cycle.

A preprint of this paper can be downloaded as a pdf file.


2013

Latitudinal Connectivity of Ground Level Enhancement Events

N. Gopalswamy and P. Mäkelä
Proceedings of: 12th Annual International Astrophysical Conference, ASP Conference Series, ed. Q. Hu and G. Zank, in press, 2013

Abstract

We examined the source regions and coronal environment of the historical ground level enhancement (GLE) events in search of evidence for non-radial motion of the associated coronal mass ejection (CME). For the 13 GLE events that had source latitudes >30° we found evidence for possible non-radial CME motion due to deflection by large-scale magnetic structures in nearby coronal holes, streamers, or pseudo streamers. Polar coronal holes are the main source of deflection in the rise and declining phases of solar cycles. In the maximum phase, deflection by large-scale streamers or pseudo streamers overlying high-latitude filaments seems to be important. The B0 angle reduced the ecliptic distance of some GLE source regions and increased in others with the net result that the average latitude of GLE events did not change significantly. The non-radial CME motion is the dominant factor that reduces the ecliptic distance of GLE source regions, thereby improving the latitudinal connectivity to Earth. We further infer that the GLE particles must be accelerated at the nose part of the CME-driven shocks, where the shock is likely to be quasi-parallel.

A preprint of this paper can be downloaded as a pdf file.


Testing the Empirical Shock Arrival Model using Quadrature Observations

N. Gopalswamy, P. Mäkelä, H. Xie, and S. Yashiro
Space Weather, in press, 2013

Abstract

The empirical shock arrival (ESA) model was developed based on quadrature data from Helios (in-situ) and P-78 (remote-sensing) to predict the Sun-Earth travel time of coronal mass ejections (CMEs) [Gopalswamy et al. 2005a]. The ESA model requires earthward CME speed as input, which is not directly measurable from coronagraphs along the Sun-Earth line. The Solar Terrestrial Relations Observatory (STEREO) and the Solar and Heliospheric Observatory (SOHO) were in quadrature during 2010 - 2012, so the speeds of Earth-directed CMEs were observed with minimal projection effects. We identified a set of 20 full halo CMEs in the field of view of SOHO that were also observed in quadrature by STEREO. We used the earthward speed from STEREO measurements as input to the ESA model and compared the resulting travel times with the observed ones from L1 monitors. We find that the model predicts the CME travel time within about 7.3 hours, which is similar to the predictions by the ENLIL model. We also find that CME-CME and CME-coronal hole interaction can lead to large deviations from model predictions.

A preprint of this paper can be downloaded as a pdf file.


A Study of Coronal Holes Observed by SOHO/EIT and the Nobeyama Radio Heliograph

Sachiko AKIYAMA, Nat GOPALSWAMY, Seiji YASHIRO, and Pertti MÄKELÄ
Publications of the Astronomical Society of Japan, accepted

Abstract

Coronal holes (CHs) are areas of reduced emission in EUV and X-ray images that show bright patches of microwave enhancements (MEs) related to magnetic network junctions inside the CHs. A clear correlation between the CH size and the solar wind (SW) speed is well known, but we have less information about the relationship between MEs and other CH and SW properties. We study the characteristics of 21 equatorial CHs associated with corotating interaction regions (CIRs) during 1996 to 2005. Our CHs are divided into two groups according to the intensity of the associated geomagnetic storms: Dst ≤ -100 nT (10 events) and > -100 nT (11 events). Using EUV 284 Å images obtained by SOHO/EIT and 17 GHz microwave images obtained by the Nobeyama Radio Heliograph (NoRH), we find a linear correlation not only between the maximum SW speed and the area of EUV CH (r = 0.62) but also between the maximum SW speed and the area of the ME (r = 0.79). We also compared the EUV CH areas with and without an overlapping ME. The area of the CHs with an ME is better correlated with the SW speed (r = 0.71) than the area of those without an ME (r = 0.36). Therefore, the radio ME may play an important role in understanding the origin of SW.

A preprint of this paper can be downloaded as a pdf file.


Obscuration of Flare Emission by an Eruptive Prominence

Nat Gopalswamy and Seiji Yashiro
Publications of the Astronomical Society of Japan, accepted

Abstract

We report on the eclipsing of microwave flare emission by an eruptive prominence from a neighboring region as observed by the Nobeyama Radioheliograph at 17 GHz. The obscuration of the flare emission appears as a dimming feature in the microwave flare light curve. We use the dimming feature to derive the temperature of the prominence and the distribution of heating along the length of the filament. We find that the prominence is heated to a temperature above the quiet Sun temperature at 17 GHz. The duration of the dimming is the time taken by the eruptive prominence in passing over the flaring region. We also find evidence for the obscuration in EUV images obtained by the Solar and Heliospheric Observatory (SOHO) mission.

A preprint of this paper can be downloaded as a pdf file.


CMEs and Active Regions on the Sun

Grzegorz Michalek and Seiji Yashiro
Advances in Space Research, 52, 521, 2013

Abstract

The relationship between active regions (ARs) and coronal mass ejections (CMEs) is studied. For this purpose a statistical analysis of 694 CMEs associated with ARs was carried out. We considered the relationship between properties of the CMEs and ARs characterized using the McIntosh classification. We demonstrated that CMEs are likely to be launched from ARs in the mature phase of their evolution when they have complex magnetic field. The fastest and halo CMEs can be ejected only from the most complex ARs (when an AR is a bipolar group of spots with large asymmetric penumbras around the main spot with many smaller spots in the group). We also showed that the wider events have a tendency to originate from uncomplicated magnetic structures. This tendency was used for estimation of the real angular widths of the halo CMEs. The probability of launching of fast CMEs increases together with increase of the complexity and size of ARs. The widest, but slow, CMEs originate from the simplest magnetic structure which are still able to produce CMEs. Our results could be useful for forecasting of space weather.

A preprint of this paper can be downloaded as a pdf file.


Topical Issue in Solar Physics: Flux-rope Structure of Coronal Mass Ejections- Preface

N. Gopalswamy, T. Nieves-Chinchilla, M. Hidalgo, J. Zhang, P. Riley, L. van Driel- Gesztelyi, and C.H. Mandrini
Solar Physics, 284, 1, 2013

Abstract

This Topical Issue of Solar Physics, devoted to the study of flux-rope structure in coronal mass ejections (CMEs), is based on two Coordinated Data Analysis Workshops (CDAWs) held in 2010 (20 - 23 September in Dan Diego, California, USA) and 2011 (September 5-9 in Alcala, Spain). The primary purpose of the CDAWs was to address the question: Do all CMEs have flux rope structure? There are 18 papers om this topical issue, including this preface.

A preprint of this paper can be downloaded as a pdf file.


Post-Eruption Arcades and Interplanetary Coronal Mass Ejections

Yashiro, S., Gopalswamy, N., Mäkelä, P., and, Akiyama, S.
Solar Physics, 284, 5, 2013

Abstract

We compare the temporal and spatial properties of posteruption arcades (PEAs) associated with coronal mass ejections (CMEs) at the Sun that end up as magnetic cloud (MC) and non-MC events in the solar wind. We investigate the length, width, area, tilt angle, and formation time of the PEAs associated with 22 MC and 29 non-MC events and we find no difference between the two populations. According to current ideas on the relation between flares and CMEs, the PEA is formed together with the CME flux-rope structure by magnetic reconnection. Our results indicate that at the Sun flux ropes form during CMEs in association with both MC and non-MC events; however, for non-MC events the flux-rope structure is not observed in the interplanetary space because of the geometry of the observation, i.e. the location of the spacecraft when the structure passes through it.

A preprint of this paper can be downloaded as a pdf file.


The First Ground Level Enhancement Event of Solar Cycle 24: Direct Observation of Shock Formation and Particle Release Heights

N. Gopalswamy, H. Xie, S. Akiyama, S. Yashiro, I. G. Usoskin, and J. M. Davila
Astrophys. J. Lett., 765, L30, 2013

Abstract

We report on the 2012 May 17 Ground Level Enhancement (GLE) event, which is the first of its kind in Solar Cycle 24. This is the first GLE event to be fully observed close to the surface by the Solar Terrestrial Relations Observatory (STEREO) mission. We determine the coronal mass ejection (CME) height at the start of the associated metric type II radio burst (i.e., shock formation height) as 1.38 Rs (from the Sun center). The CME height at the time of GLE particle release was directly measured from a STEREO image as 2.32 Rs, which agrees well with the estimation from CME kinematics. These heights are consistent with those obtained for cycle-23 GLEs using back-extrapolation. By contrasting the 2012 May 17 GLE with six other non-GLE eruptions from well-connected regions with similar or larger flare size and CME speed, we find that the latitudinal distance from the ecliptic is rather large for the non-GLE events due to a combination of non-radial CME motion and unfavorable solar B0 angle, making the connectivity to Earth poorer. We also find that the coronal environment may play a role in deciding the shock strength.

A preprint of this paper can be downloaded as a pdf file.


Erratum: "Behavior of Solar Cycles 23 and 24 Revealed by Microwave Observations" (2012, ApJ, 750, L42)

N. Gopalswamy, S. Yashiro, P. Mäkelä, G. Michalek, K. Shibasaki, and D. H. Hathaway
The Astrophysical Journal, in press

A preprint of this paper can be downloaded as a pdf file.


Near-Sun Flux-Rope Structure of CMEs

H. Xie, N. Gopalswamy, O.C. St. Cyr
Solar Physics, 284, 47, 2013

Abstract

We have used the Krall flux-rope model (Krall and St. Cyr, Astrophys. J. 2006, 657, 1740) (KFR) to fit 23 magnetic cloud (MC)-CMEs and 30 non-cloud ejecta (EJ)-CMEs in the Living With a Star (LWS) Coordinated Data Analysis Workshop (CDAW) 2011 list. The KFR-fit results shows that the CMEs associated with MCs (EJs) have been deflected closer to (away from) the solar disk center (DC), likely by both the intrinsic magnetic structures inside an active region (AR) and ambient magnetic structures (e.g. nearby ARs, coronal holes, and streamers, etc.). The mean absolute propagation latitudes and longitudes of the EJ-CMEs (18°, 11° ) were larger than those of the MC-CMEs (11°, 6°) by 7° and 5°, respectively. Furthermore, the KFR-fit widths showed that the MC-CMEs are wider than the EJ-CMEs. The mean fitting face-on width and edge-on width of the MC-CMEs (EJ-CMEs) were 87 (85)° and 70 (63)°, respectively. The deflection away from DC and narrower angular widths of the EJ-CMEs have caused the observing spacecraft to pass over only their flanks and miss the central flux-rope structures. The results of this work support the idea that all CMEs have a flux-rope structure.

A preprint of this paper can be downloaded as a pdf file.


Coronal Hole Influence on the Observed Structure of Interplanetary CMEs

P. Mäkelä, N. Gopalswamy, H. Xie, A. A. Mohamed, S. Akiyama, S. Yashiro
Solar Physics, 284, 59, 2013

Abstract

We report on the coronal hole (CH) influence on the 54 magnetic cloud (MC) and non-MC associated coronal mass ejections (CMEs) selected for studies during the Coordinated Data Analysis Workshops (CDAWs) focusing on the question if all CMEs are flux ropes. All selected CMEs originated from source regions located between longitudes 15E - 15W. Xie, Gopalswamy, and St. Cyr (2013, Solar Phys., doi:10.1007/s11207-012-0209-0) found that these MC and non-MC associated CMEs are on average deflected towards and away from the Sun-Earth line, respectively. We used a CH influence parameter (CHIP) that depends on the CH area, average magnetic field strength, and distance from the CME source region to describe the influence of all on-disk CHs on the erupting CME. We found that for CHIP values larger than 2.6 G the MC and non-MC events separate into two distinct groups where MCs (non-MCs) are deflected towards (away) from the disk center. Division into two groups was also observed when the distance to the nearest CH was less than 3.2 × 105 km. At CHIP values less than 2.6 G or at distances of the nearest CH larger than 3.2 × 105 km.

A preprint of this paper can be downloaded as a pdf file.


Height of Shock Formation in the Solar Corona Inferred from Observations of Type II Radio Bursts and Coronal Mass Ejections

N. Gopalswamy, H. Xie, P. Mäkelä, S. Yashiro, S. Akiyama, W. Uddin., A. K. Srivastava, N. C. Joshi, R. Chandra, P. K. Manoharan, K. Mahalakshmi, V. C. Dwivedi, R. Jain and A. K. Awasthi, N. V. Nitta, M. J. Aschwanden, D. P. Choudhary
Adv. Space Res., 51, 1981, 2013

Abstract

Employing coronagraphic and EUV observations close to the solar surface made by the Solar Terrestrial Relations Observatory (STEREO) mission, we determined the heliocentric distance of coronal mass ejections (CMEs) at the starting time of associated metric type II bursts. We used the wave diameter and leading edge methods and measured the CME heights for a set of 32 metric type II bursts from solar cycle 24. We minimized the projection effects by making the measurements from a view that is roughly orthogonal to the direction of the ejection. We also chose image frames close to the onset times of the type II bursts, so no extrapolation was necessary. We found that the CMEs were located in the heliocentric distance range from 1.20 to 1.93 solar radii (Rs), with mean and median values of 1.43 and 1.38 Rs, respectively. We conclusively find that the shock formation can occur at heights substantially below 1.5 Rs. In a few cases, the CME height at type II onset was close to 2 Rs. In these cases, the starting frequency of the type II bursts was very low, in the range 25 - 40 MHz, which confirms that the shock can also form at larger heights. The starting frequencies of metric type II bursts have a weak correlation with the measured CME/shock heights and are consistent with the rapid decline of density with height in the inner corona.

A preprint of this paper can be downloaded as a pdf file.


2012

The Solar Connection of Enhanced Heavy Ion Charge States in the Interplanetary Medium: Implications for the Flux-rope Structure of CMEs

N. Gopalswamy, P. Mäkelä, S. Akiyama, H. Xie, S. Yashiro, and A. A. Reinard
Solar Physics, 284, 17, 2013

Abstract

We investigated a set of 54 interplanetary coronal mass ejection (ICME) events whose solar sources are very close to the disk center (within ±15 degrees from the central meridian). The ICMEs consisted of 23 magnetic cloud (MC) events and 31 non-MC events. Our analyses suggest that the MC and non-MC ICMEs have more or less the same eruption characteristics at the Sun in terms of soft X-ray flares and CMEs. Both types have significant enhancements in charge states, although the non-MC structures have slightly lower levels of enhancement. The overall duration of charge state enhancement is also considerably smaller than that than that in MCs as derived from solar wind plasma and magnetic signatures. We find very good correlation between the Fe and O charge state measurements and the flare properties such as soft X-ray flare intensity and flare temperature for both MCs and non-MCs. These observations suggest that both MC and non-MC ICMEs are likely to have a flux-rope structure and the unfavorable observational geometry may be responsible for the appearance of non-MC structures at 1 AU. We do not find any evidence for active region expansion resulting in ICMEs lacking a flux rope structure because the mechanism of producing high charge states and the flux rope structure at the Sun is the same for MC and non-MC events.

A preprint of this paper can be downloaded as a pdf file.


Determination of the Heliospheric Radial Magnetic Field from the Standoff Distance of a CME-driven Shock Observed by the STEREO Spacecraft

Watanachak Poomvises, Nat Gopalswamy, Seiji Yashiro, Ryun Young Kwon, Oscar Olmedo
The Astrophysical Journal, Vol 758, Issue 2, p118, 2012

Abstract

We report on the determination of radial magnetic field strength in the heliocentric distance range from 6 to 120 solar radii (R) using data from Coronagraph 2 (COR2) and Heliospheric Imager I (HI1) instruments onboard the Solar Terrestrial Relations Observatory (STEREO) spacecraft following the standoff-distance method of Gopalswamy and Yashiro (2011). We measured the shock standoff distance of the 2008 April 5 Coronal Mass Ejection (CME) and determined the flux rope curvature by fitting the 3D shape of the CME using the Graduated Cylindrical Shell (CGS) model. The radial magnetic field strength is computed from the Alfven speed and the density of the ambient medium. We also compare the derived magnetic field strength with in-situ measurements made by the Helios spacecraft, which measured the magnetic field at the heliocentric distance range from 60 to 215 R. We found that the radial magnetic field strength decreases from 28 mG at 6 R to 0.17 mG at 120 R. In addition, we found that the radial profile can be described by a power law.

A preprint of this paper can be downloaded as a pdf file.


3D structure and evolution of EUV bright points observed by STEREO/SECCHI/EUVI

Ryun-Young Kwon, Jongchul Chae, Joseph M. Davila, Jie Zhang, Yong-Jae Moon, Watanachak Poomvises, & Shaela I. Jones
The Astrophysical Journal, Vol 757, Issue 2, p167, 2012

Abstract

We unveil the three-dimensional structure of quiet-Sun EUV bright points and its temporal evolution by applying the triangulation method to images taken by SECCHI/EUVI on board STEREO twin spacecraft. For this study we examine the heights and lengths, as components of three-dimensional structure of EUV bright points and their temporal evolutions. Among them we present three bright points which show three distinct patterns of evolution. We show that the three distinct types (decreasing, increasing, and steady in height and length) of EUV bright points are consistent with photospheric motions (converging, diverging, and shearing, respectively) of their underlying magnetic flux concentrations. They all have multi-temperature loop systems in which hot loops are overlying cooler loops with a strong correlation between height and length. Both flux emergence and cancellation occur during their lifetimes: flux emergence is dominant in the initial phase and flux cancellation becomes significant when the radiance flux of a bright point reaches its maximum. Our results suggest that magnetic flux emergences may play an important role in magnetic reconnection and EUV bright points are semi-circular and multi-thermal structures connecting the two opposite magnetic poles, formed by magnetic reconnection.

A preprint of this paper can be downloaded as a pdf file.


A Tell-Tale Sign of a Wimpy Solar Cycle: the First GLE Event of Solar Cycle 24

N. Gopalswamy
INQUIRIES OF HEAVEN, WEDNESDAY, AUGUST 29, 2012 DAY 8, IAU General assembly, 2012

Abstract

The current cycle 24 has produced only one GLE event so far on May 17, 2012, whereas cycle 23 had produced five of the 16 GLEsin the frst 4.5 years. The Sun is already in its solar maximum phase, which means it did not produce any GLE event during its rise phase. The lone GLE event is consistent with a weak cycle 24: the sunspot number peaked at 97 compared to 170 in cycle 23, indicating that cycle 24 is 40% weaker.

A preprint of this paper can be downloaded as a pdf file.


Observations of CMEs and Models of the Eruptive Corona

Nat Gopalswamy
the Proc. Solar Wind 13, 2012

Abstract

Current theoretical ideas on the internal structure of CMEs suggest that a flux rope is central to the CME structure, which has considerable observational support both from remote-sensing and in-situ observations. The flux-rope nature is also consistent with the post-eruption arcades with high-temperature plasma and the charge states observed within CMEs arriving at Earth. The model involving magnetic loop expansion to explain CMEs without flux ropes is not viable because it contradicts CME kinematics and flare properties near the Sun. The global picture of CMEs becomes complete if one includes the shock sheath to the CSHKP model.

A preprint of this paper can be downloaded as a pdf file.


Radio-loud CMEs from the disk center lacking shocks at 1 AU

N. Gopalswamy, P. Mäkelä, S. Akiyama, S. Yashiro, H. Xie, R. J. MacDowall, and M. L. Kaiser
Journal of Geophysical Research, 2012, 117, A08, CiteID:A08106.

Abstract

A coronal mass ejection (CME) associated with a type II burst and originating close to the center of the solar disk typically results in a shock at Earth in 2-3 days and hence can be used to predict shock arrival at Earth. However, a significant fraction (about 28%) of such CMEs producing type II bursts were not associated with shocks at Earth. We examined a set of 21 type II bursts observed by the Wind/WAVES experiment at decameter-hectometric (DH) wavelengths that had CME sources very close to the disk center (within a central meridian distance of 30 degrees), but did not have a shock at Earth. We find that the near-Sun speeds of these CMEs average to ~644 km/s, only slightly higher than the average speed of CMEs associated with radio-quiet shocks. However, the fraction of halo CMEs is only ~30%, compared to 54% for the radio-quiet shocks and 91% for all radio-loud shocks. We conclude that the disk-center radio-loud CMEs with no shocks at 1 AU are generally of lower energy and they drive shocks only close to the Sun and dissipate before arriving at Earth. There is also evidence for other possible processes that lead to the lack of shock at 1 AU: (i) overtaking CME shocks merge and one observes a single shock at Earth, and (ii) deflection by nearby coronal holes can push the shocks away from the Sun-Earth line, such that Earth misses these shocks. The probability of observing a shock at 1 AU increases rapidly above 60% when the CME speed exceeds 1000 km/s and when the type II bursts propagate to frequencies below 1 MHz.

A preprint of this paper can be downloaded as a pdf file.


Energetic Particle and Other Space Weather Events of Solar Cycle 24

Nat Gopalswamy
American Institute of Physics Conference Proceedings, Proc. 11th International Astrophysical Conference, edited by Q. Hu, G. Li, G. P. Zank, G. Fry, X. Ao, and J. Adams, 2012 (in press)

Abstract

We report on the space weather events of solar cycle 24 in comparison with those during a similar epoch in cycle 23. We find major differences in all space weather events: solar energetic particles, geomagnetic storms, and interplanetary shocks. Dearth of ground level enhancement (GLE) events and major geomagnetic storms during cycle 24 clearly standout. The space weather events seem to reflect the less frequent solar eruptions and the overall weakness of solar cycle 24.

A preprint of this paper can be downloaded as a pdf file.


Deflections of fast coronal mass ejections and the properties of associated solar energetic partivle events

S, W. Kahler, S. Akiyama, and N. Gopalswamy
The Astrophysical Journal, 754, 100, 2012

Abstract

The onset times and peak intensities of solar energetic particle (SEP) events at Earth have long been thought to be influenced by the open magnetic fields of coronal holes (CHs). The original idea was that a CH lying between the solar SEP source region and the magnetic footpoint of the 1 AU observer would result in a delay in onset and/or a decrease in the peak intensity of that SEP event. Recently, Gopalswamy et al. showed that CHs near coronal mass ejection (CME) source regions can deflect fast CMEs from their expected trajectories in space, explaining the appearance of driverless shocks at 1 AU from CMEs ejected near solar central meridian (CM). This suggests that SEP events originating in CME-driven shocks may show variations attributable to CH deflections of the CME trajectories. Here, we use a CH magnetic force parameter to examine possible effects of CHs on the timing and intensities of 41 observed gradual E ~ 20 MeV SEP events with CME source regions within 20° of CM. We find no systematic CH effects on SEP event intensity profiles. Furthermore, we find no correlation between the CME leading-edge measured position angles and SEP event properties, suggesting that the widths of CME-driven shock sources of the SEPs are much larger than the CMEs. Independently of the SEP event properties, we do find evidence for significant CME deflections by CH fields in these events.

See also NASA/ADS.


Properties of Ground Level Enhancement Events and the Associated Solar Eruptions during Solar Cycle 23

N. Gopalswamy, H. Xie, S. Yashiro, S. Akiyama, P. Mäkelä, and I. G. Usoskin
Space Science Reviews, in press, 2012

Abstract

Solar cycle 23 witnessed the most complete set of observations of coronal mass ejections (CMEs) associated with the Ground Level Enhancement (GLE) events. We present an overview of the observed properties of the GLEs and those of the two associated phenomena, viz., flares and CMEs, both being potential sources of particle acceleration. Although we do not find a striking correlation between the GLE intensity and the parameters of flares and CMEs, the solar eruptions are very intense involving X-class flares and extreme CME speeds (average ~2000 km/s). An M7.1 flare and a 1200 km/s CME are the weakest events in the list of 16 GLE events. Most (80%) of the CMEs are full halos with the three non-halos having widths in the range 167 to 212 degrees. The active regions in which the GLE events originate are generally large: 1290 msh (median 1010 msh) compared to 934 msh (median: 790 msh) for SEP-producing active regions. For accurate estimation of the CME height at the time of metric type II onset and GLE particle release, we estimated the initial acceleration of the CMEs using flare and CME observations. The initial acceleration of GLE-associated CMEs is much larger (by a factor of 2) than that of ordinary CMEs (2.3 km/s/s vs.1 km/s/s). We confirmed the initial acceleration for two events for which CME measurements are available in the inner corona. The GLE particle release is delayed with respect to the onset of all electromagnetic signatures of the eruptions: type II bursts, low frequency type III bursts, soft X-ray flares and CMEs. The presence of metric type II radio bursts some 17 min (median: 16 min; range: 3 to 48 min) before the GLE onset indicates shock formation well before the particle release. The release of GLE particles occurs when the CMEs reach an average height of ~3.09 Rs (median: 3.18 Rs; range: 1.71 to 4.01 Rs) for well-connected events (source longitude in the range W20 - W90). For poorly connected events, the average CME height at GLE particle release is ~66% larger (mean: 5.18 Rs; median: 4.61 Rs; range: 2.75 - 8.49 Rs). The longitudinal dependence is consistent with shock accelerations because the shocks from poorly connected events need to expand more to cross the field lines connecting to an Earth observer. On the other hand, the CME height at metric type II burst onset has no longitudinal dependence because electromagnetic signals do not require magnetic connectivity to the observer. For several events, the GLE particle release is very close to the time of first appearance of the CME in the coronagraphic field of view, so we independently confirmed the CME height at particle release. The CME height at metric type II burst onset is in the narrow range 1.29 to 1.8 Rs, with mean and median values of 1.53 and 1.47 Rs. The CME heights at metric type II burst onset and GLE particle release correspond to the minimum and maximum in the Alfven speed profile. The increase in CME speed between these two heights suggests an increase in Alfvenic Mach number from 2 to 3. The CME heights at GLE particle release are in good agreement with those obtained from the velocity dispersion analysis (Reames, 2009a,b) including the source longitude dependence. We also discuss the implications of the delay of GLE particle release with respect to complex type III bursts by ~18 min (median: 16 in; range: 2 to 44 min) for the flare acceleration mechanism. A similar analysis is also performed on the delay of particle release relative to the hard X-ray emission.

A preprint of this paper can be downloaded as a pdf file.


The Relationship Between the Expansion Speed and Radial Speed of CMEs Confirmed Using Quadrature Observations of the 2011 February 15 CME

Nat Gopalswamy, Pertti Mäkelä, Seiji Yashiro, and Joseph M. Davila
Sun and Geosphere, 2012, 7(1), pp 7-11.

Abstract

It is difficult to measure the true speed of Earth-directed CMEs from a coronagraph located along the Sun-Earth line because of the occulting disk. However, the expansion speed (the speed with which the CME appears to spread in the sky plane) can be measured by such a coronagraph. In order to convert the expansion speed to radial speed (which is important for space weather applications) one can use an empirical relationship between the two that assumes an average width for all CMEs. If we have the width information from quadrature observations, we can confirm the relationship between expansion and radial speeds derived by Gopalswamy et al. (2009a). The STEREO spacecraft were in qudrature with SOHO (STEREO-A ahead of Earth by 87 deg and STEREO-B 94 deg behind Earth) on 2011 February 15, when a fast Earth-directed CME occurred. The CME was observed as a halo by the Large-Angle and Spectrometric Coronagraph (LASCO) on board SOHO. The sky-plane speed was measured by SOHO/LASCO as the expansion speed, while the radial speed was measured by STEREO-A and STEREO-B. In addition, STEREO-A and STEREO-B images provided the width of the CME, which is unknown from Earth view. From the SOHO and STEREO measurements, we confirm the relationship between the expansion speed (Vexp) and radial speed (Vrad) derived previously from geometrical considerations (Gopalswamy et al. 2009a): Vrad =1/2 (1 + cot w)Vexp, where w is the half width of the CME. STEREO-B images of the CME, we found that CME had a full width of 76 deg, so w = 38 deg. This gives the relation as Vrad = 1.14 Vexp. From LASCO observations, we measured Vexp = 897 km/s, so we get the radial speed as 1023 km/s. Direct measurement of radial speed yields 945 km/s (STEREO-A) and 1058 km/s (STEREO-B). These numbers are different only by 7.6% and 3.4% (for STEREO-A and STEREO-B, respectively) from the computed value.

A preprint of this paper can be downloaded from NASA/ADS.


Behavior of Solar Cycles 23 and 24 Revealed by Microwave Observations

N. Gopalswamy, S. Yashiro, P. Mäkelä, G. Michalek, K. Shibasaki, and D. H. Hathaway
The Astrophysical Journal Letters, 750, L42, 2012

Abstract

Using magnetic and microwave butterfly diagrams, we compare the behavior of solar polar regions to show that (i) the polar magnetic field and the microwave brightness temperature during the solar minimum substantially diminished during the cycle 23/24 minimum compared to the 22/23 minimum. (ii) The polar microwave brightness temperature (b) seems to be a good proxy for the underlying magnetic field strength (B). The analysis indicates a relationship, B = 0.0067Tb - 70, where B is in G and Tb in K. (iii) Both the brightness temperature and the magnetic field strength show north-south asymmetry most of the time except for a short period during the maximum phase. (iv) The rush-to-the-pole phenomenon observed in the prominence eruption activity seems to be complete in the northern hemisphere as of March 2012. (v) The decline of the microwave brightness temperature in the north polar region to the quiet-Sun levels and the sustained prominence eruption activity poleward of 60°N suggest that solar maximum conditions have arrived at the northern hemisphere. The southern hemisphere continues to exhibit conditions corresponding to the rise phase of solar cycle 24.

A preprint of this paper can be downloaded as a pdf file.


Understanding Shock Dynamics in the Inner Heliosphere with Modeling and Type II Radio Data: the 2010-04-03 event

H. Xie, D. Odstrcil, L. Mays, O. C. St. Cyr, N. Gopalswamy, and H. Cremades
Journal of Geophysical Research, 2012, 117, A4, CiteID A04105

Abstract

The 2010 April 03 solar event was studied 4 using observations from STEREO SECCHI, SOHO LASCO, and Wind kilometric Type II data (kmTII) combined with WSA-Cone-ENLIL model simulations performed at the Community Coordinated Modeling Center (CCMC). In particular, we identified the origin of the coronal mass ejection (CME) using STEREO EUVI and SOHO EIT images. A flux-rope model was fit to the SECCHI A and B, and LASCO images to determine the CME's direction, size, and actual speed. J-maps from STEREO COR2/HI-1/HI-2 and simulations from CCMC were used to study the formation and evolution of the shock in the inner heliosphere. In addition, we also studied the time-distance profile of the shock propagation from kmTII radio burst observations. The J-maps together with in-situ data from the Wind spacecraft provided an opportunity to validate the simulation results and the kmTII prediction. Here we report on a comparison of two methods of predicting interplanetary shock arrival time: the ENLIL model and the kmTII method; and investigate whether or not using the ENLIL model density improves the kmTII prediction. We found that the ENLIL model predicted the kinematics of shock evolution well. The shock arrival times (SAT) and linear-fit shock velocities in the ENLIL model agreed well with those measurements in the J-maps along both the CME leading edge and the Sun-Earth line. The ENLIL model also reproduced most of the large scale structures of the shock propagation and gave the SAT prediction at Earth with an er ror of ~1 ± 7 hours. The kmTII method predicted the SAT at Earth with an error of ~15 hours when using n0 = 4.16 cm-3, the ENLIL model plasma density near Earth; but it improved to ~2 hours when using n0 = 6.64 cm-3, the model density near the CME leading edge at 1 AU.

A preprint of this paper can be downloaded as a pdf file.


Initiation of Coronal Mass Ejection and Associated Flare Caused by Helical Kink Instability Observed by SDO/AIA

Pankaj Kumar, K.-S. Cho, S.-C. Bong, Sung-Hong Park, and Y. H. Kim
The Astrophysical Journal, Volume 746, Issue 1, article id. 67 (2012)

Abstract

In this paper, we present multiwavelength observations of helical kink instability as a trigger of a coronal mass ejection (CME) which occurred in active region NOAA 11163 on 2011 February 24. The CME was associated with an M3.5 limb flare. High-resolution observations from the Solar Dynamics Observatory/Atmospheric Imaging Assembly suggest the development of helical kink instability in the erupting prominence, which implies a flux rope structure of the magnetic field. A brightening starts below the apex of the prominence with its slow rising motion (~100 km s-1) during the activation phase. A bright structure, indicative of a helix with ~3-4 turns, was transiently formed at this position. The corresponding twist of ~6π-8π is sufficient to generate the helical kink instability in a flux rope according to recently developed models. A slowly rising blob structure was subsequently formed at the apex of the prominence, and a flaring loop was observed near the footpoints. Within 2 minutes, a second blob was formed in the northern prominence leg. The second blob erupts (like a plasmoid ejection) with the detachment of the northern prominence leg, and flare intensity maximizes. The first blob at the prominence apex shows rotational motion in the counterclockwise direction in the plane of sky, interpreted as the unwinding motion of a helix, and it also erupts to give the CME. RHESSI hard X-ray (HXR) sources show the two footpoint sources and a loop-top source during the flare. We found RHESSI HXR flux, soft X-ray flux derivative, and CME acceleration in the low corona correlate well, which is in agreement with the standard flare model (CSHKP). We also discuss the possible role of ballooning as well as torus instabilities in driving the CME. We conclude that the CME and flare were triggered by the helical kink instability in a flux rope and accelerated mainly by the torus instability.

A preprint of this paper can be downloaded from NASA/ADS.


2011

Geometry of the 20 November 2003 magnetic cloud

Katsuhide Marubashi, Kyung-Suk Cho, Yeon-Han Kim, Yong-Deuk Park, and Sung-Hong Park
Journal of Geophysical Research, Volume 117, Issue A1, CiteID A01101

Abstract

This study is an attempt to find a coherent interpretation of the link between the 20 November 2003 magnetic cloud (MC) and its solar source. Most previous studies agree on the orientation of the MC, but the orientation is nearly perpendicular to the axis of the post-eruption arcade (PEA) or the orientation of the neutral line in the solar source region. We first determine the geometry of this MC by fitting methods with both torus and cylinder models. Three possible geometries are obtained, which can reproduce the observed magnetic field variations associated with the MC, one from the cylinder fit and two from the torus fit. The cylinder fit gives the MC orientation with a tilt of a large angle (-60°) from the ecliptic plane and nearly perpendicular to the PEA axis, being similar to those from previous studies. In contrast, two torus fit results give the MC axis with tilt angles less than 20° from the ecliptic plane. The two torus results correspond to the spacecraft encounter with the eastern flank of the flux rope loop (model A) and the western flank of the loop (model B), respectively. In either case, the orientation of the loop around the apex is nearly parallel to the PEA as observed by the SOHO/extreme ultraviolet imaging telescope instrument in the most plausible solar source region of a halo coronal mass ejection (CME), which appeared in the field of view of Large Angle and Spectrometric Coronagraph (LASCO) C2 at 08:50 UT, 18 November 2003. The magnetic helicity of the PEA region is positive in agreement with the helicity of the MC. The 3-D reconstruction from the Solar Mass Ejection Imager data shows that the main part of the ejected plasma expands mainly to the west of the Sun-Earth line. Thus, we reach the most straightforward interpretation of the link between the MC and its solar source as follows. The MC was created in association with the launch of the CME that was first observed by the LASCO C2 at 08:50 UT, 18 November 2003, and propagated through interplanetary space with its orientation almost unchanged. The spacecraft encountered the eastern flank of the loop as described by model A.

See also NASA/ADS.


Coronal Magnetic Field Measurement from EUV Images made by the Solar Dynamics Observatory

Nat Gopalswamy, Nariaki Nitta, Sachiko Akiyama, Pertti Mäkelä, and Seiji Yashiro
The Astrophysical Journal, 744, 72, 2012

Abstract

By measuring the geometrical properties of the coronal mass ejection (CME) flux rope and the leading shock observed on 2010 June 13 by the Solar Dynamics Observatory (SDO) mission's Atmospheric Imaging Assembly (AIA) we determine the Alfvén speed and the magnetic field strength in the inner corona at a heliocentric distance of ~ 1.4 Rs. The basic measurements are the shock standoff distance (ΔR) ahead of the CME flux rope, the radius of curvature of the flux rope (Rc), and the shock speed. We first derive the Alfvénic Mach number (M) using the relationship, ΔR/Rc = 0.81[(γ-1)M2 + 2]/[(γ+1)(M2-1)], where γ is the only parameter that needed to be assumed. For γ =4/3, the Mach number declined from 3.7 to 1.5 indicating shock weakening within the field of view of the imager. The shock formation coincided with the appearance of a type II radio burst at a frequency of ~300 MHz (harmonic component), providing an independent confirmation of the shock. The shock compression ratio derived from the radio dynamic spectrum was found to be consistent with that derived from the theory of fast mode MHD shocks. From the measured shock speed and the derived Mach number, we found the Alfvén speed to increase from ~140 km/s to 460 km/s over the distance range 1.2 to 1.5 Rs. By deriving the upstream plasma density from the emission frequency of the associated type II radio burst, we determined the coronal magnetic field to be in the range 1.3 to 1.5 G. The derived magnetic field values are consistent with other estimates in a similar distance range. This work demonstrates that the EUV imagers, in the presence of radio dynamic spectra, can be used as coronal magnetometers.

A preprint of this paper can be downloaded from NASA/ADS.


Factors Affecting The Intensity of Solar Energetic Particle Events

Nat Gopalswamy
in Proc. Tenth annual Astrophysics Conf., ed. J. Heerikhuisen, G. Li, and G. Zank, American Institute of Physics, in press.

Abstract

This paper updates the influence of environmental and source factors of shocks driven by coronal mass ejections (CMEs) that are likely to influence the solar energetic particle (SEP) events. The intensity variation due to CME interaction reported in [1] is confirmed by expanding the investigation to all the large SEP events of solar cycle 23. The large SEP events are separated into two groups, one associated with CMEs running into other CMEs, and the other with CMEs running into the ambient solar wind. SEP events with CME interaction generally have a higher intensity. New possibilities such as the influence of coronal holes on the SEP intensity are also discussed. For example, the presence of a large coronal hole between a well-connected eruption and the solar disk center may render the shock poorly connected because of the interaction between the CME and the coronal hole. This point is illustrated using the 2004 December 3 SEP event delayed by about 12 hours from the onset of the associated CME. There is no other event at the Sun that can be associated with the SEP onset. This event is consistent with the possibility that the coronal hole interaction influences the connectivity of the CMEs that produce SEPs, and hence the intensity of the SEP event.

A preprint of this paper can be downloaded as a pdf file.


Earth-Affecting Solar Causes Observatory (EASCO): A mission at the Sun-Earth L5

Nat Gopalswamy, Joseph M. Davila, Frédéric Auchère, Jesper Schou, Clarence Korendike, Albert Shih, Janet C. Johnston, Robert J. MacDowall, Milan Maksimovic, Edward Sittler, Adam Szabo, Richard Wesenberg, Suzanne Vennerstrom, and Bernd Heber
SPIE (Society of Photo-Optical Instrumentation Engineers), accepted

Abstract

Coronal mass ejections (CMEs) and corotating interaction regions (CIRs) as well as their source regions are important because of their space weather consequences. The current understanding of CMEs primarily comes from the Solar and Heliospheric Observatory (SOHO) and the Solar Terrestrial Relations Observatory (STEREO) missions, but these missions lacked some key measurements: STEREO did not have a magnetograph; SOHO did not have in-situ magnetometer. SOHO and other imagers such as the Solar Mass Ejection Imager (SMEI) located on the Sun-Earth line are also not well-suited to measure Earth-directed CMEs. The Earth-Affecting Solar Causes Observatory (EASCO) is a proposed mission to be located at the Sun-Earth L5 that overcomes these deficiencies. The mission concept was recently studied at the Mission Design Laboratory (MDL), NASA Goddard Space Flight Center, to see how the mission can be implemented. The study found that the scientific payload (seven remote-sensing and three in-situ instruments) can be readily accommodated and can be launched using an intermediate size vehicle; a hybrid propulsion system consisting of a Xenon ion thruster and hydrazine has been found to be adequate to place the payload at L5. Following a 2-year transfer time, a 4-year operation is considered around the next solar maximum in 2025.

A preprint of this paper can be downloaded as a pdf file.


The Strength and Radial Profile of Coronal Magnetic Field from the Standoff Distance of a CME-driven Shock

Nat Gopalswamy and Seiji Yashiro
The Astrophysical Journal Letters, 736, L17, 2011

Abstract

We determine the coronal magnetic field strength in the heliocentric distance range 6 to 23 solar radii (Rs) by measuring the shock standoff distance and the radius of curvature of the flux rope during the 2008 March 25 coronal mass ejection (CME) imaged by white-light coronagraphs. Assuming the adiabatic index, we determine the Alfven Mach number, and hence the Alfven speed in the ambient medium using the measured shock speed. By measuring the upstream plasma density using polarization brightness images, we finally get the magnetic field strength upstream of the shock. The estimated magnetic field decreases from ~48 mG around 6 Rs to 8 mG at 23 Rs. The radial profile of the magnetic field can be described by a power law in agreement with other estimates at similar heliocentric distances.

A preprint of this paper can be downloaded as a pdf file.


Coronal Mass Ejections and Solar Radio Emissions

N. Gopalswamy
Accepted for Publication in the book Planetary Radio Emissions VII, Eds. Rucker, H. O., W. S. Kurth, P. Louarn, G. Fischer, Austrian Academy of Sciences Press, Vienna, in press (2011)

Abstract

Three types of low-frequency nonthermal radio bursts are associated with coronal mass ejections (CMEs): Type III bursts due to accelerated electrons propagating along open magnetic field lines, type II bursts due to electrons accelerated in shocks, and type IV bursts due to electrons trapped in post-eruption arcades behind CMEs. This paper presents a summary of results obtained during solar cycle 23 primarily using the white-light coronagraphic observations from the Solar Heliospheric Observatory (SOHO) and the WAVES experiment on board Wind.

A preprint of this paper can be downloaded as a pdf file.


Energetic Storm Particle Events in CME-driven Shocks

P. Mäkelä, N. Gopalswamy, S. Akiyama, H. Xie, and S. yashiro
Journal of Geophysical Research, 2011, 116, A8, CiteID A08101

Abstract

We investigate the variability in the occurrence of energetic storm particle (ESP) events associated with shocks driven by coronal mass ejections (CMEs). The interplanetary shocks were detected during the period from 1996 to 2006. First we analyze the CME properties near the Sun. The CMEs with an ESP-producing shock are faster (<VCME> = 1088 km/s) than those driving shocks without an ESP event (<VCME> = 771 km/s) and have a larger fraction of halo CMEs (67% vs. 38%). The Alfvénic Mach numbers of shocks with an ESP event are on average 1.6 times higher than those of shocks without. We also contrast the ESP event properties and frequency in shocks with and without a type II radio burst by dividing the shocks into radio-loud (RL) and radio-quiet (RQ) shocks, respectively. The shocks seem to be organized into a decreasing sequence by the energy content of the CMEs: RL shocks with an ESP event are driven by the most energetic CMEs, followed by RL shocks without an ESP event, then RQ shocks with and with- out an ESP event. The ESP events occur more often in RL shocks than in RQ shocks: 52% of RL shocks and only ∼32% of RQ shocks produced an ESP event at proton energies above 1.8 MeV; in the keV energy range the ESP frequencies are 80% and 65%, respectively. Electron ESP events were detected in 19% of RQ shocks and 39% of RL shocks. In addition we find that (1) ESP events in RQ shocks are less intense than those in RL shocks; (2) RQ shocks with ESP events are predominately quasi-perpendicular shocks; and (3) their solar sources are located slightly to the east of the central meridian; (4) ESP event sizes show a modest positive correlation with the CME and shock speeds. The observation that RL shocks tend to produce more frequently ESP events with larger particle flux increases than RQ shocks, emphasizes the importance of type II bursts in identifying solar events prone to producing high particle fluxes in the near-Earth space. However the trend is not definitive. If there is no type II emission, an ESP event is less likely but not absent. The variability in the probability and size of ESP events most likely reflects differences in the shock formation in the low corona and changes in the properties of the shocks as they propagate through interplanetary space, and the escape efficiency of accelerated particles from the shock front.

A preprint of this paper can be downloaded as a pdf file.


Universal Heliophysical Processes

Gopalswamy, N.
In: The Sun, the Solar Wind, and the Heliosphere, ed. M. P. Miralles and J. Sanchez Almeida, IAGA Special Sopron Book Series, Vol 4, Chapter 2, Springer, pp 9-20, 2011 DOI: 10.1007/978/90-481-9787-3_2

Abstract

The physical processes in the heliospace are a direct consequence of the influenced by Sun's mass and electromagnetic emissions. There has been enormous progress in studying these processes since the dawn of the space age half a century ago. The heliospace serves as a great laboratory to study numerous physical processes, using the vast array of ground and space-based measurements of various physical quantities. The observational capabilities collectively form the Great Observatory to make scientific investigations not envisioned by individual instrument teams. The International Heliophysical Year (IHY) program has been promoting scientific investigations on the universality of physical processes such as shocks, particle acceleration, dynamo, magnetic reconnection, magnetic flux ropes, plasma-neutral matter interactions, turbulence, and so on. This paper highlights scientific deliberations on these and related topics that took place during the IAGA sessionon "Universal Heliophysical Processes" in Sopron, Hungary. The session featured several invited and contributed papers that focused on observations, theory and modeling of the universal heliophysical processes.

A preprint of this paper can be downloaded as a pdf file.


Earth-Affecting Solar Causes Observatory (EASCO): A Potential International Living with a Star Mission from Sun-Earth L5

N. Gopalswamy, J.Davila, O. C. St. Cyr, E. Sittler, F. Auchere, T. Duvall, T. Hoeksema, M. Maksimovic, R. MacDowall, A. Szabo, and M. Collier
Journal of Atmospheric and Solar-Terrestrial Physics, 73, 658, 2011

Abstract

This paper describes the scientific rationale for an L5 mission and a partial list of key scientific instruments the mission should carry. The L5 vantage point provides an unprecedented view of the solar disturbances and their solar sources that can greatly advance the science behind space weather. A coronagraph and a heliospheric imager at L5 will be able to view CMEs broadsided, so space speed of the Earth-directed CMEs can be measured accurately and their radial structure discerned. In addition, an inner coronal imager and a magnetograph from L5 can give advance information on active regions and coronal holes that will soon rotate on to the solar disk. Radio remote sensing at low frequencies can provide information on shock-driving CMEs, the most dangerous of all CMEs. Coordinated helioseismic measurements from the Sun-Earth line and L5 provide information on the physical conditions at the base of the convection zone, where solar magnetism originates. Finally, in situ measurements at L5 can provide information on the large-scale solar wind structures (corotating interaction regions (CIRs)) heading towards Earth that potentially result in adverse space weather.

A preprint of this paper can be downloaded as a pdf file.


Low-frequency type III radio bursts and solar energetic particle events

N. Gopalswamy and P. Mäkelä
Central European Astrophysics Bulletin, 2011, 35, pp71-82

Abstract

Complex type III bursts at low-frequencies (>14 MHz) are thought to indicate large solar energetic particle (SEP) events. We analyzed six complex type III bursts from the same active region, one of which was not accompanied by a SEP event. This event was accompanied by a fast and wide coronal mass ejection (CME), but lacked a type II burst and an interplanetary shock. When we examined the evolution and the magnetic configuration of the active region, we did not find anything peculiar. The lowest frequency of type III emission occurred at the local plasma frequency in the vicinity of the Wind spacecraft that observed the type III, which confirms that the magnetic connectivity of the source region was good. We conclude that the lack of SEPs is due to the lack of production rather than due to poor magnetic connectivity. We also show that neither the type III burst duration nor the burst intensity was able to distinguish between SEP and non-SEP events. The lack of SEP event can be readily explained under the shock-acceleration paradigm, but not under the flare-acceleration paradigm.

A preprint of this paper can be downloaded as a pdf file.


MAXIMUM CORONAL MASS EJECTION SPEED AS AN INDICATOR OF SOLAR AND GEOMAGNETIC ACTIVITIES

A. Kilcik, V. B. Yurchyshyn, V. Abramenko, P. R. Goode, N. Gopalswamy, A. Ozguc, and J. P. Rozelot
The Astrophysical Journal, 727, 44, 2011

Abstract

We investigate the relationship between the monthly averaged maximal speeds of coronal mass ejections (CMEs), international sunspot number (ISSN), and the geomagnetic Dst and Ap indices covering the 1996-2008 time interval (solar cycle 23). Our new findings are as follows. (1) There is a noteworthy relationship between monthly averaged maximum CME speeds and sunspot numbers, Ap and Dst indices. Various peculiarities in the monthly Dst index are correlated better with the fine structures in the CME speed profile than that in the ISSN data. (2) Unlike the sunspot numbers, the CME speed index does not exhibit a double peak maximum. Instead, the CME speed profile peaks during the declining phase of solar cycle 23. Similar to the Ap index, both CME speed and the Dst indices lag behind the sunspot numbers by several months. (3) The CME number shows a double peak similar to that seen in the sunspot numbers. The CME occurrence rate remained very high even near the minimum of the solar cycle 23, when both the sunspot number and the CME average maximum speed were reaching their minimum values. (4) A well-defined peak of the Ap index between 2002 May and 2004 August was co-temporal with the excess of the mid-latitude coronal holes during solar cycle 23. The above findings suggest that the CME speed index may be a useful indicator of both solar and geomagnetic activities. It may have advantages over the sunspot numbers, because it better reflects the intensity of Earth-directed solar eruptions.

A preprint of this paper can be downloaded from NASA/ADS.


2010

An empirical model for prediction of geomagnetic storms using initially observed CME parameters at the Sun

R.-S. Kim, K.-S. Cho, Y.-J. Moon, M. Dryer, J. Lee, Y. Yi, K.-H. Kim, H. Wang, Y.-D. Park, and Yong Ha Kim
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, A12108, doi:10.1029/2010JA015322, 2010

Abstract

In this study, we discuss the general behaviors of geomagnetic storm strength associated with observed parameters of coronal mass ejection (CME) such as speed (V) and earthward direction (D) of CMEs as well as the longitude (L) and magnetic field orientation (M) of overlaying potential fields of the CME source region, and we develop an empirical model to predict geomagnetic storm occurrence with its strength (gauged by the Dst index) in terms of these CME parameters. For this we select 66 halo or partial halo CMEs associated with M-class and X-class solar flares, which have clearly identifiable source regions, from 1997 to 2003. After examining how each of these CME parameters correlates with the geoeffectiveness of the CMEs, we find several properties as follows: (1) Parameter D best correlates with storm strength Dst; (2) the majority of geoeffective CMEs have been originated from solar longitude 15W, and CMEs originated away from this longitude tend to produce weaker storms; (3) correlations between Dst and the CME parameters improve if CMEs are separated into two groups depending on whether their magnetic fields are oriented southward or northward in their source regions. Based on these observations, we present two empirical expressions for Dst in terms of L, V, and D for two groups of CMEs, respectively. This is a new attempt to predict not only the occurrence of geomagnetic storms, but also the storm strength (Dst) solely based on the CME parameters.

See also NASA/ADS.


Coronal Mass Ejections: a Summary of Recent Results

N. Gopalswamy
Proceedings of the 20th Slovak National Solar Physics Workshop, ed. I. Dorotovic, Slovak Central Observatory, pp. 108 - 130, 2010

Abstract

Coronal mass ejections (CMEs) have been recognized as the most energetic phenomenon in the heliosphere, deriving their energy from the stressed magnetic fields on the Sun. This paper summarizes the properties of CMEs and highlights some of the recent results on CMEs. In particular, the morphological, physical, and kinematic properties of CMEs are summarized. The CME consequences in the heliosphere such as interplanetary shocks, type II radio bursts, energetic particles, geomagnetic storms, and cosmic ray modulation are discussed.

A preprint of this paper can be downloaded as a pdf file.


Ground level enhancement events of Solar cycle 23

Gopalswamy, N., Xie, H., Yashiro, S., and Usoskin, I.
Indian Journal of Radio & Space Physics 39, pp. 240-248, 2010

Abstract

Ground level enhancement (GLE) events, typically in the GeV energy range, are the most energetic of solar energetic particle (SEP) events with the protons penetrating Earth¡Çs neutral atmosphere. During solar cycle 23, sixteen GLE events were observed with excellent data coverage of associated solar eruptions. The source of these GLE particles has been examined in this paper using white light observations of coronal mass ejections, type II radio bursts, and soft X-ray flares. It has been shown that the GLE events are consistent with shock acceleration in every single case. While the possibility of the presence of a flare component during GLE events cannot be ruled out, it can be definitely said that a shock component is present in all the GLE events. During 18 April 2001 GLE event, the source was located ~30 degrees behind the west limb, which is too far away from the magnetic field lines connecting Earth and hence the flare component can be ruled out. Also the presence of interplanetary shocks associated with the GLEs is shown using radio and in-situ observations.

A preprint of this paper can be downloaded as a pdf file.


Long-duration Low-frequency Type III Bursts and Solar Energetic Particle Events

N. Gopalswamy and P. Mäkelä
Astrophysical Journal Letters 721, L62-L66, 2010

Abstract

We analyzed the coronal mass ejections (CMEs), flares, and type II radio bursts associated with a set of three complex, long-duration, low-frequency (<14 MHz) type III bursts from active region 10588 in 2004 April. The durations were measured at 1 and 14 MHz using data from Wind/WAVES and were well above the threshold value (>15 minutes) normally used to define these bursts. One of the three type III bursts was not associated with a type II burst, which also lacked a solar energetic particle (SEP) event at energies >25 MeV. The 1 MHz duration of the type III burst (28 minutes) for this event was near the median value of type III durations found for gradual SEP events and ground level enhancement events. Yet, there was no sign of an SEP event. On the other hand, the other two type III bursts from the same active region had similar duration but were accompanied by WAVES type II bursts; these bursts were also accompanied by SEP events detected by SOHO/ERNE. The CMEs for the three events had similar speeds, and the flares also had similar size and duration. This study suggests that the occurrence of a complex, long-duration, low-frequency type III burst is not a good indicator of an SEP event.

See also NASA/ADS.


Large-Scale Solar Eruptions

Gopalswamy, N.
in Heliophysical Processes, ed. N. Gopalswamy, S. S. Hasan, A. Ambastha, Springer, 53 - 71, 2010

Abstract

This chapter provides an over view of coronal mass ejections (CMEs) and the associated flares including statistical properties, associated phenomena (solar energetic particles, interplanetary shocks, geomagnetic storms), and their heliospheric consequences.

See also NASA/ADS.


2009

Relation between Type II Bursts and CMEs Inferred from STEREO Observations

N. Gopalswamy, W.T. Thompson, J.M. Davila, M.L. Kaiser, S. Yashiro, P. Mäkelä, G. Michalek, J.-L. Bougeret & R.A. Howard
Solar Phys. 259, 227, 2009

Abstract

The inner coronagraph (COR1) of the Solar Terrestrial Relations Observatory (STEREO) mission has made it possible to observe CMEs in the spatial domain overlapping with that of the metric type II radio bursts. The type II bursts were associated with generally weak flares (mostly B and C class soft X-ray flares), but the CMEs were quite energetic. Using CME data for a set of type II bursts during the declining phase of solar cycle 23, we determine the CME height when the type II bursts start, thus giving an estimate of the heliocentric distance at which CME-driven shocks form. This distance has been determined to be ~1.5Rs (solar radii), which coincides with the distance at which the Alfven speed profile has a minimum value.We also use type II radio observations from STEREO/WAVES and Wind/WAVES observations to show that CMEs with moderate speed drive either weak shocks or no shock at all when they attain a height where the Alfven speed peaks (~3Rs - 4Rs). Thus the shocks seem to be most efficient in accelerating electrons in the heliocentric distance range of 1.5Rs to 4Rs. By combining the radial variation of the CME speed in the inner corona (CME speed increase) and interplanetary medium (speed decrease) we were able to correctly account for the deviations from the universal drift-rate spectrum of type II bursts, thus confirming the close physical connection between type II bursts and CMEs. The average height (~1.5 Rs) of STEREO CMEs at the time of type II bursts is smaller than that (2.2 Rs) obtained for SOHO (Solar and Heliospheric Observatory) CMEs. We suggest that this may indicate, at least partly, the density reduction in the corona between the maximum and declining phases, so a given plasma level occurs closer to the Sun in the latter phase. In two cases, there was diffuse shock-like feature ahead of the main body of the CME, indicating a standoff distance of 1-2 Rs by the time the CME left the LASCO FOV.

A preprint of this paper can be downloaded as a pdf file.


Erratum to "Solar sources and geospace consequences of interplanetary magnetic clouds observed during solar cycle 23 - Paper1" [J. Atmos.Sol.-Terr. Phys.70(2-4) (2008) 245-253]

N. Gopalswamy, S.Akiyama S.Yashiro, G.Michalek, R.P.Lepping,
J. of Atmospheric and Solar-Terrestrial Physics, Vol. 71, pp. 1005-1009, 2009

Abstract

The paper, "Solar sources and geospace consequences of interplanetary magnetic clouds observed during solar cycle 23" (Gopalswamy et al., 2008) contains some unfortunate errors in one of the Figures (Fig.4) and the Electronic Table. This note is to correct these errors.

See also NASA/ADS.


A Catalog of Halo Coronal Mass Ejections from SOHO

N. Gopalswamy, S. Yashiro, G. Michalek, H. Xie, P. Mäkelä, A. Vourlidas, R. A. Howard
Sun and Geosphere, Vol.4 - No.1 - 2009, in press

Abstract

Coronal mass ejections (CMEs) that appear to surround the occulting disk of the observing coronagraph are known as halo CMEs. Halos constitute a subset of energetic CMEs that have important heliospheric consequences. Here we describe an on-line catalog that contains all the halo CMEs that were identified in the images obtained by the Solar and Heliospheric Observatory (SOHO) mission's Large Angle and Spectrometric Coronagraph (LASCO) since 1996. Until the end of 2007, some 396 halo CMEs were recorded. For each halo CME, we identify the solar source (heliographic coordinates), the soft X-ray flare importance, and the flare onset time. From the sky-plane speed measurements and the solar source information we obtain the space speed of CMEs using a cone model. In addition to the description of the catalog (
http://cdaw.gsfc.nasa.gov/CME_list/HALO/halo.html), we summarize the statistical properties of the halo CMEs. We confirm that halo CMEs are twice faster than ordinary CMEs and are associated with major flares on the average. We also compared the annual rate of halo CMEs with that obtained by automatic detection methods and found that most of these methods have difficulty in identifying full halo CMEs.

A preprint of this paper can be downloaded as a pdf file.


The Sun and Earth's space environment

Gopalswamy, N.
Proceeding of the 2009 International Conference on Space Science and Communication, 26-27 October 2009, Port Dickson, Negeri Sembilan, Malaysia, pp 5-10, DOI: 10.1109/ICONSPACE.2009.5352679.

Abstract

Earth's space environment is closely controlled by solar variability over various time scales. Solar variability is characterized by its output in the form of mass and electromagnetic output. Solar mass emission also interacts with mass entering into the heliosphere in the form of cosmic rays and neutral material. This paper provides an overview of how the solar variability affects Earth's space environment.

See also IEEE Xplore.


CME link to the geomagnetic storms

Nat Gopalswamy
in Solar and Stellar Variability: Impact on Earth and Planets, Proceedings IAU Symposium No. 264, 2009, H. Andrei, A. Kosovichev & J.-P. Rozelot, eds.

Abstract

The coronal mass ejection (CME) link to geomagnetic storms stems from the southward component of the interplanetary magnetic field contained in the CME flux ropes and in the sheath between the flux rope and the CME-driven shock. A typical storm-causing CME is characterized by (i) high speed, (ii) large angular width (mostly halos and partial halos), and (iii)solar source location close to the central meridian. For CMEs originating at larger central meridian distances, the storms are mainly caused by the sheath field. Both the magnetic and energy contents of the storm-producing CMEs can be traced to the magnetic structure of active regions and the free energy stored in them.

A preprint of this paper can be downloaded as a pdf file.


Solar Sources of "Driverless" Interplanetary Shocks

N. Gopalswamy, P. Mäkelä, H. Xie, S. Akiyama and S.Yashiro
Solar Wind 12, American Institute of Physics Conference Proceedings, in press, 2009

Abstract

We identify the solar sources of a large number of interplanetary (IP) shocks that do not have a discernible driver as observed by spacecraft along the Sun-Earth line. At the Sun, these "driverless" shocks are associated with fast and wide CMEs. Most of the CMEs were also driving shocks near the Sun, as evidenced by the association of IP type II radio bursts. Thus, all these shocks are driven by CMEs and they are not blast waves. Normally limb CMEs produce driverless shocks at 1 AU. But some disk-center CMEs also result in driverless shocks because of deflection by nearby coronal holes. We estimate the angular deflection to be in the range 20 deg - 60 deg. We also compared the influence of nearby coronal holes on a set of CMEs that resulted in magnetic clouds. The influence is nearly three times larger in the case of driverless shocks, confirming the large deflection required.

A preprint of this paper can be downloaded as a pdf file.


Coronal Mass Ejections from Sunspot and non-Sunspot Regions

N. Gopalswamy, S. Akiyama, S. Yashiro, and P. Mäkelä
To appear in "Magnetic Coupling between the Interior and the Atmosphere of the Sun", eds. S. S. Hasan and R. J. Rutten, Astrophysics and Space Science Proceedings, Springer-Verlag, Heidelberg, Berlin, 2009.

Abstract

Coronal mass ejections (CMEs) originate from closed magnetic field regions on the Sun, which are active regions and quiescent filament regions. The energetic populations such as halo CMEs, CMEs associated with magnetic clouds, geoeffective CMEs, CMEs associated with solar energetic particles and interplanetary type II radio bursts, and shock-driving CMEs have been found to originate from sunspot regions. The CME and flare occurrence rates are found to be correlated with the sunspot number, but the correlations are significantly weaker during the maximum phase compared to the rise and declining phases. We suggest that the weaker correlation results from high-latitude CMEs from the polar crown filament regions that are not related to sunspots.

A preprint of this paper can be downloaded from arXiv.


CME Interaction with Coronal Holes and their Interplanetary Consequences

N. Gopalswamy, P. Mäkelä, H. Xie, S. Akiyama, and S. Yashiro
J. Geophys. Res., Vol. 114, A00A22, doi:10.1029/2008JA013686, 2009

Abstract

A significant number of interplanetary (IP) shocks (~17%) during cycle 23 were not followed by drivers. The number of such "driverless" shocks steadily increased with the solar cycle with 15%, 33%, and 52% occurring in the rise, maximum, and declining phase of the solar cycle. The solar sources of 15% of the driverless shocks were very close the central meridian of the Sun (within ~15o), which is quite unexpected. More interestingly, all the driverless shocks with their solar sources near the solar disk center occurred during the declining phase of solar cycle 23. When we investigated the coronal environment of the source regions of driverless shocks, we found that in each case there was at least one coronal hole nearby suggesting that the coronal holes might have deflected the associated coronal mass ejections (CMEs) away from the Sun-Earth line. The presence of abundant low-latitude coronal holes during the declining phase further explains why CMEs originating close to the disk center mimic the limb CMEs, which normally lead to driverless shocks due to purely geometrical reasons. We also examined the solar source regions of shocks with drivers. For these, the coronal holes were located such that they either had no influence on the CME trajectories, or they deflected the CMEs towards the Sun-Earth line. We also obtained the open magnetic field distribution on the Sun by performing a potential field source surface extrapolation to the corona. It was found that the CMEs generally move away from the open magnetic field regions. The CME-coronal hole interaction must be widespread in the declining phase, and may have a significant impact on the geoeffectiveness of CMEs.

A preprint of this paper can be downloaded as a pdf file.


THE SOHO/LASCO CME CATALOG

N. Gopalswamy, S. Yashiro, G. Michalek, G. Stenborg, A. Vourlidas, S. Freeland, and R. Howard
Earth, Moon, and Planets, Volume 104, Issue 1, Page 295, 2009

Abstract

Coronal mass ejections (CMEs) are routinely identified in the images of the solar corona obtained by the Solar and Heliospheric Observatory (SOHO) mission's Large Angle and Spectrometric Coronagraph (LASCO) since 1996. The identified CMEs are measured and their basic attributes are cataloged in a data base known as the SOHO/LASCO CME Catalog. The Catalog also contains digital data, movies, and plots for each CME, so detailed scientific investigations can be performed on CMEs and the related phenomena such as flares, radio bursts, solar energetic particle events, and geomagnetic storms. This paper provides a brief description of the Catalog and summarizes the statistical properties of CMEs obtained using the Catalog. Data products relevant to space weather research and some CME issues that can be addressed using the Catalog are discussed. The URL of the Catalog is: http://cdaw.gsfc.nasa.gov/CME_list.

A preprint of this paper can be downloaded as a pdf file.


Halo Coronal Mass Ejections and Geomagnetic Storms

Nat Gopalswamy
Earth, Planets and Space (EPS), 61, 1-3, 2009

Abstract

In this letter, I show that the discrepancies in the geoeffectiveness of halo coronal mass ejections (CMEs) reported in the literature arise due to the varied definitions of halo CMEs used by different authors. In particular, I show that the low geoeffectiveness rate is a direct consequence of including partial halo CMEs. The geoeffectiveness of partial halo CMEs is lower because they are of low speed and likely to make a glancing impact on Earth.

See alos NASA/ADS.


EUV Wave Reflection from a Coronal Hole

N. Gopalswamy, S. Yashiro, M. Temmer, J. Davila, W. T. Thompson, S. Jones, R. T. J. McAteer, J.-P. Wuelser, S. Freeland, and R. A. Howard
Astrophys. J. Lett., 2009, Vol 691, L123-L127.

Abstract

We report on the detection of EUV wave reflection from a coronal hole, as observed by the Solar TErrestrial RElations Observatory (STEREO) mission. The EUV wave was associated with a coronal mass ejection (CME) erupting near the disk center. It was possible to measure the kinematics of the reflected waves for the first time. The reflected waves were generally slower than the direct wave. One of the important implications of the wave reflection is that the EUV transients are truly a wave phenomenon. The EUV wave reflection has implications for CME propagation, especially during the declining phase of the solar cycle when there are many low-latitude coronal holes.

A preprint of this paper can be downloaded as a pdf file.


2008

Large Geomagnetic Storms Associated with Limb Halo Coronal Mass Ejections

Nat Gopalswamy, Seiji Yashiro, Hong Xie, Sachiko Akiyama, and Pertti Makela
Advances in Geosciences, Vol. 21, 71, 2008

Abstract

Solar cycle 23 witnessed the observation of hundreds of halo coronal mass ejections (CMEs), thanks to the high dynamic range and extended field of view of the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO) mission. More than two thirds of halo CMEs originating on the front side of the Sun have been found to be geoeffective (Dst =< -50 nT). The delay time between the onset of halo CMEs and the peak of ensuing geomagnetic storms has been found to depend on the solar source location (Gopalswamy et al., 2007). In particular, limb halo CMEs (source longitude > 45deg) have a 20% shorter delay time on the average. It was suggested that the geomagnetic storms due to limb halos must be due to the sheath portion of the interplanetary CMEs (ICMEs) so that the shorter delay time can be accounted for. We confirm this suggestion by examining the sheath and ejecta portions of ICMEs from Wind and ACE data that correspond to the limb halos. Detailed examination showed that three pairs of limb halos were interacting events. Geomagnetic storms following five limb halos were actually produced by other disk halos. The storms followed by four isolated limb halos and the ones associated with interacting limb halos, were all due to the sheath portions of ICMEs.

A preprint of this paper can be downloaded as a pdf file.


Large Geomagnetic Storms: Introduction to Special Section

N. Gopalswamy
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, A00A00, doi:10.1029/2008JA014026, 2009

Abstract

Solar cycle 23 witnessed the accumulation of rich data sets that reveal various aspects of geomagnetic storms in unprecedented detail both at the Sun where the storm causing disturbances originate and in geospace where the effects of the storms are directly felt. During two recent coordinated data analysis workshops (CDAWs) the large geomagnetic storms (Dst < -100 nT) of solar cycle 23 were studied in order to understand their solar, interplanetary, and geospace connections. This special section grew out of these CDAWs with additional contributions relevant to these storms. Here I provide a brief summary of the results presented in the special section.

See also NASA/ADS.


Comment on "Prediction of the 1-AU arrival times of CMEassociated interplanetary shocks: Evaluation of an empirical interplanetary shock propagation model" by K.-H. Kim et al.

N. Gopalswamy and H. Xie
J. Geophys. Res., 113, A10105, doi:10.1029/2008JA013030, 2008

Abstract

Recently, Kim et al. [2007] (hereinafter referred to as KMC) have evaluated the empirical shock arrival (ESA) model and found only about 60% of the observed shocks arrived within +-12 h of the model prediction. They also found the deviations of shock travel times from the ESA model strongly correlate with the CME initial speeds (VCME), suggesting that the constant interplanetary (IP) acceleration used in the ESA model may not be applicable to all CMEs. KMC further concluded that faster CMEs decelerate and slower CMEs accelerate more than that what is considered in the ESA model. We point out that such systematic deviations in arrival time arise owing to projection effects.

See also NASA/ADS.


Conservation of open solar magnetic flux and the floor in the heliospheric magnetic field

M. J. Owens, N. U. Crooker, N. A. Schwadron, T. S. Horbury, S. Yashiro, H. Xie, O. C. St. Cyr, and N. Gopalswamy
Geophys. Res. Lett., 35, L20108, doi:10.1029/2008GL035813., 2008

Abstract

The near-Earth heliospheric magnetic field intensity, |B|, exhibits a strong solar cycle variation, but returns to the same "floor" value each solar minimum. The current minimum, however, has seen |B| drop below previous minima, bringing in to question the existence of a floor, or at the very least requiring a re-assessment of its value. In this study we assume heliospheric flux consists of a constant open flux component and a time-varying contribution from CMEs. In this scenario, the true floor is |B| with zero CME contribution. Using observed CME rates over the solar cycle, we estimate the "no-CME" |B| floor at 4.0 N1 0.3 nT, lower than previous floor estimates and below |B| observed this solar minimum. We speculate that the drop in |B| observed this minimummay be due to a persistently lowerCME rate than the previous minimum, though there are large uncertainties in the supporting observational data.

See also NASA/ADS.


Synthetic radio maps of CME-driven shocks below 4 solar radii heliocentric distance

J. M. Schmidt and N. Gopalswamy,
J. Geophys. Res., 113, A08104, doi:10.1029/2007JA013002, 2008

Abstract

We present 2 1/2 D numerical MagnetoHydroDynamic (MHD) simulations of coronal mass ejections (CMEs) in conjunction with plasma simulations of radio emission from the CME-driven shocks. The CME-driven shock extends to an almost spherical shape during the temporal evolution of the CME. Our plasma simulations can reproduce the dynamic spectra of coronal type II radio bursts, with the frequency drift rates corresponding to the shock speeds. We find further, that the CME-driven shock is an effective radio emitter at metric wavelengths, when the CME has reached a heliocentric distance of about two solar radii (R). We apply our simulation results to explain the radio images of type II bursts obtained by radio heliographs, in particular to the bananashaped images of radio sources associated with fast CMEs.

See also NASA/ADS.


THE EXPANSION AND RADIAL SPEEDS OF CORONAL MASS EJECTIONS

N. Gopalswamy, A. Dal Lago, S. Yashiro and S. Akiyama
Cent. Eur. Astrophys. Bull. vol (2008) 1, 10, in press

Abstract

We show the relation between radial (Vrad) and expansion (Vexp) speeds of coronal mass ejections (CMEs) depends on the CME width. As CME width increases, Vrad=Vexp decreases from a value >1 to <1. For widths approaching 180 deg, the ratio approaches 0 if the cone has a at base, while it approaches 0.5 if the base has a bulge (ice cream cone). The speed difference between the limb and disk halos and the spherical expansion of superfast CMEs can be explained by the width dependence.

A preprint of this paper can be downloaded as a pdf file.


Major Solar Flares without Coronal Mass Ejections

N. Gopalswamy, S. Akiyama and S. Yashiro
Universal Heliophysical Processes, Proc. IAU Symposium 257, N. Gopalswamy, & D. Webb, eds. in press (2008)

Abstract

We examine the source properties of X-class soft X-ray flares that were not associated with coronal mass ejections (CMEs). All the flares were associated with intense microwave bursts implying the production of high energy electrons. However, most (85%) of the flares were not associated with metric type III bursts, even though open field lines existed in all but two of the active regions. The X-class flares seem to be truly confined because there was no material ejection (thermal or nonthermal) away from the flaring region.

A preprint of this paper can be downloaded as a pdf file.


Statistical Relationship between Solar Flares and Coronal Mass Ejections

Seiji Yashiro and Nat Gopalswamy
Universal Heliophysical Processes, Proc. IAU Symposium 257, N. Gopalswamy, & D. Webb, eds. in press (2008)

Abstract

We report on the statistical relationships between solar flares and coronal mass ejections (CMEs) observed during 1996-2007 inclusively. We used soft X-ray flares observed by the Geo- stationary Operational Environmental Satellite (GOES) and CMEs observed by the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO) mission. Main results are (1) the CME association rate increases with flare's peak flux, fluence, and duration, (2) the difference between flare and CME onsets shows a Gaussian distribution with the standard deviation sigma = 17 min (sigma = 15 min) for the first (second) order extrapolated CME onset, (3) the most frequent flare site is under the center of the CME span, not near one leg (outer edge) of the CMEs, (4) a good correlation was found between the flare fluence versus the CME kinetic energy. Implications for flare-CME models are discussed.

A preprint of this paper can be downloaded as a pdf file.


Solar connections of geoeffective magnetic structures

N. Gopalswamy
J. of Atmospheric and Solar-Terrestrial Physics (2008), doi:10.1016/j.jastp.2008.06.010

Abstract

Coronal mass ejections (CMEs) and high-speed solar wind streams (HSS) are two solar phenomena that produce large-scale structures in the interplanetary (IP) medium. CMEs evolve into interplanetary CMEs (ICMEs) and the HSS result in corotating interaction regions (CIRs) when they interact with preceding slow solar wind. This paper summarizes the properties of these structures and describes their geoeffectiveness. The primary focus is on the intense storms of solar cycle 23 because this is the first solar cycle during which simultaneous, extensive, and uniform data on solar, IP, and geospace phenomena exist. After presenting illustrative examples of coronal holes and CMEs, I discuss the internal structure of ICMEs, in particular the magnetic clouds (MCs). I then discuss how the magnetic field and speed correlate in the sheath and cloud portions of ICMEs. CME speed measured near the Sun also has significant correlations with the speed and magnetic field strengths measured at 1 AU. The dependence of storm intensity on MC, sheath, and CME properties is discussed pointing to the close connection between solar and IP phenomena. I compare the delay time between MC arrival at 1 AU and the peak time of storms for the cloud and sheath portions and show that the internal structure of MCs leads to the variations in the observed delay times. Finally, I examine the variation of solar-source latitudes of IP structures as a function of the solar cycle and find that they have to be very close to the disk center.

See also NASA/ADS.


A comparison of coronal mass ejections identified by manual and automatic methods

S. Yashiro, G. Michalek, and N. Gopalswamy
Annales Geophysicae, 26, 3103-3112, 2008

Abstract

Coronal mass ejections (CMEs) are related to many phenomena (e.g. flares, solar energetic particles, geomagnetic storms), thus compiling of event catalogs is important for a global understanding these phenomena. CMEs have been identified manually for a long time, but in the SOHO era, automatic identification methods are being developed. In order to clarify the advantage and disadvantage of the manual and automatic CME catalogs, we examined the distributions of CME properties listed in the CDAW (manual) and CACTus (automatic) catalogs. Both catalogs have a good agreement on the wide CMEs (width>120 deg) in their properties, while there is a significant discrepancy on the narrow CMEs (width<30 deg): CACTus has a larger number of narrow CMEs than CDAW. We carried out an event-byevent examination of a sample of events and found that the CDAW catalog have missed many narrow CMEs during the solar maximum. Another significant discrepancy was found on the fast CMEs (speed>1000 km/s): the majority of the fast CDAW CMEs are wide and originate from low latitudes, while the fast CACTus CMEs are narrow and originate from all latitudes. Event-by-event examination of a sample of events suggests that CACTus has a problem on the detection of the fast CMEs.

See also NASA/ADS.


Type II Radio Emission and Solar Energetic Particle Events

Nat Gopalswamy
Accepted for publication in the Proc. of 7th IGPP Astrophysics Conference, Kauai, HI, March 7-13, 2008.

Abstract

Type II radio bursts, solar energetic particle (SEP) events, and interplanetary (IP) shocks all have a common cause, viz., fast and wide (speed > 900 km/s and width > 60 deg) coronal mass ejections (CMEs). Deviations from this general picture are observed as (i) lack of type II bursts during many fast and wide CMEs and IP shocks, (ii) slow CMEs associated with type II bursts and SEP events, and (iii) lack of SEP events during many type II bursts. I examine the reasons for these deviations. I also show that ground level enhancement (GLE) events areconsistent with shock acceleration because a type II burst is present in every event well beforethe release of GLE particles and SEPs at the Sun.

A preprint of this paper can be downloaded as a pdf file.


Coronal Mass Ejections, Type II Radio Bursts, and Solar Energetic Particle Events in the SOHO Era

N. Gopalswamy, S. Yashiro, S. Akiyama, P. Makela, H. Xie, M. Kaiser, R. Howard, and J.-L. Bougeret
Annales Geophysicae, Vol. 26, Issue 10, p. 3033, 2008

Abstract

Using the extensive and uniform data on coronal mass ejections (CMEs), solar energetic particle (SEP) events, and type II radio bursts during the SOHO era, we discuss how the CME properties such as speed, width and solar-source longitude decide whether CMEs are associated with type II radio bursts and SEP events. We discuss why some radio-quiet CMEs are associated with small SEP events while some radio-loud CMEs are not associated with SEP events. We conclude that either some fast and wide CMEs do not drive shocks or they drive weak shocks that do not produce significant levels of particle acceleration. We also infer that the Alfven speed in the corona and near-Sun interplanetary medium ranges from <200 km/s to ~1600 km/s. Radio-quiet fast and wide CMEs are also poor SEP producers and the association rate of type II bursts and SEP events steadily increases with CME speed and width (i.e., energy). If we consider western hemispheric CMEs, the SEP association rate increases linearly from ~30% for 800 km/s CMEs to 100% for >1800 km/s. Essentially all type II bursts in the decametre-hectometric (DH) wavelength range are associated with SEP events once the source location on the Sun is taken into account. This is a significant result for space weather applications, because if a CME originating from the western hemisphere is accompanied by a DH type II burst, there is a high probability that it will produce an SEP event.

See also NASA/ADS.


Radio-Quiet Fast and Wide Coronal Mass Ejections

N. Gopalswamy, S. Yashiro, H. Xie, S. Akiyama, E. Aguilar-Rodriguez, M. L. Kaiser, R. A. Howard, and J.-L. Bougeret
Astrophysical Journal, Vol. 674, p. 560, 2008

Abstract

We report on the properties of radio-quiet (RQ) and radio-loud (RL) coronal mass ejections (CMEs) that are fast and wide (FW). RQ CMEs lack of type II radio bursts in the metric and decameterhectometric (DH) wavelengths. RL CMEs are associated with metric or DH type II bursts. We found that ~ 40% of the FW CMEs from 1996 to 2005 were radio quiet. The RQ CMEs had an average speed of 1117 km/s compared to 1438 km/s for the RL, bracketing the average speed of all FW CMEs (1303 km/s). The fraction of full halo CMEs (apparent width = 360 deg) was the largest for the RL CMEs (60%), smallest for the RQ CMEs (16%) and intermediate for all FW CMEs (42%). The median soft X-ray flare size for the RQ CMEs (C6.9) was also smaller than that for the RL CMEs (M3.9). About 55% of RQ CMEs were back-sided, while the frontsided ones originated close to the limb. The RL CMEs originated generally on the disk with only ~25% being backsided. The RQ FW CMEs suggest that the Alfven speed in the low-latitude outer corona can often exceed 1000 km/s and can vary over a factor of >3. None of the RQ CMEs was associated with large solar energetic particles, which is useful information for space weather applications.

A preprint of this paper can be downloaded as a pdf file.


Spatial Relationship between Solar Flares and Coronal Mass Ejections

S. Yashiro, G. Michalek, S. Akiyama, N. Gopalswamy, and R. A. Howard
Astrophysical Journal, Vol. 673, p. 1174, 2008

Abstract

We report on the spatial relationship between solar flares and coronal mass ejections (CMEs) observed during 1996-2005 inclusive. We identified 496 flare-CME pairs considering limb flares (distance from central meridian > 45 deg) with soft X-ray flare size > C3 level. The CMEs were detected by the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO). We investigated the flare positions with respect to the CME span for the events with X-class, M-class, and C-class flares separately. It is found that the most frequent flare site is at the center of the CME span for all the three classes, but that frequency is different for the different classes. Many X-class flares often lie at the center of the associated CME, while C-class flares widely spread to the outside of the CME span. The former is different from previous studies, which concluded that no preferred flare site exists. We compared our result with the previous studies and conclude that the long-term LASCO observation enabled us to obtain the detailed spatial relation between flares and CMEs. Our finding calls for a closer flare-CME relationship and supports eruption models typified by the CSHKP magnetic reconnection model.

A preprint of this paper can be downloaded from arXiv.


Solar Sources and Geospace Consequences of Interplanetary Magnetic Clouds Observed During Solar Cycle 23

N. Gopalswamy, S. Akiyama, S. Yashiro, G. Michalek, and R. P. Lepping
J. of Atmospheric and Solar-Terrestrial Physics, Vol. 70, pp. 245-253, 2008

Abstract

We present results of a statistical investigation of 99 magnetic clouds (MCs) observed during 1995-2005. The MC-associated coronal mass ejections (CMEs) are faster and wider on the average and originate within ±30 deg from the Sun center. The solar sources of MCs also followed the butter y diagram. The correlation between the magnetic field strength and speed of MCs was found to be valid over a much wider range of speeds. The number of south-north (SN) MCs was dominant and decreased with solar cycle, while the number of north-south (NS) MCs increased confrming the odd-cycle behavior. Two-thirds of MCs were geoe ective; the Dst index was highly correlated with speed and magnetic field in MC as well as their product. Many (55%) fully northward (FN) MCs were geoe ective solely due to their sheaths. The non-geoe ective MCs were slower (average speed 382 km/s), had a weaker southward magnetic field (average -5.2 nT), and occurred mostly during the rise phase of the solar activity cycle.

See also NASA/ADS.


2007

Prediction Space Weather Using an Asymmetric Cone Model

G. Michalek, N. Gopalswamy, and S. Yashiro
Solar Physics, Volume 246, Number 2, pp. 399-408

Abstract

Halo coronal mass ejections (HCMEs) are responsible of the most severe geomagnetic storms. A prediction of their geoeffectiveness and travel time to Earth's vicinity is crucial to forecast space weather. Unfortunately coronagraphic observations are subjected to projection effects and do not provide true characteristics of CMEs. Recently, Michalek (2006, Solar Phys., 237, 101) developed an asymmetric cone model to obtain the space speed, width and source location of HCMEs. We applied this technique to obtain the parameters of all front-sided HCMEs observed by the SOHO/LASCO experiment during a period from the beginning of 2001 until the end of 2002 (solar cycle 23). These parameters were applied for the space weather forecast. Our study determined that the space speeds are strongly correlated with the travel times of HCMEs within Earth's vicinity and with the magnitudes related to geomagnetic disturbances.

A preprint of this paper can be downloaded from arXiv.


Width of Radio-Loud and Radio-Quiet CMEs

G. Michalek, N. Gopalswamy, and H. Xie
Solar Physics, Volume 246, Number 2, pp. 409-414, 2007

Abstract

In the present paper we report on the difference in angular sizes between radio-loud and radio-quiet CMEs. For this purpose we compiled these two samples of events using Wind/WAVES and SOHO/LASCO observations obtained during 1996-2005. It is shown that the radio-loud CMEs are almost two times wider than the radio-quiet CMEs (considering expanding parts of CMEs). Furthermore we show that the radio-quiet CMEs have a narrow expanding bright part with a large extended diffusive structure. These results were obtained by measuring the CME widths in three different ways.

A preprint of this paper can be downloaded from arXiv.


Energetic Particles Related with Coronal and Interplanetary Shocks

N. Gopalswamy
The high energy solar corona: Waves, eruptions, particles, Lecture Notes in Physics 725, ed. K.-L. Klein and A. MacKinnon, p. 139-160, 2007

Abstract

Acceleration of electrons and ions at the Sun is discussed in the framework of CME-driven shocks. Based on the properties of coronal mass ejections associated with type II bursts at various wavelenths, the possibility of a unified approach to the type II phenomena is suggested. Two aspects of primary importance to shock accelerations are: (1) Energy of the driving CME and (2) the conditions in the medium that supports shock propagation. The high degree of overlap between CMEs associated with large solar energetic particle events and type II bursts occurring at all wavelengths underscores the importance of CME energy in driving shocks far into the interplanetary medium. Presence of preceding CMEs can alter the conditions in the ambient medium, which is shown to influence the intensity of large solar energetic particle events. Both statistical evidence and case studies are presented that underscore the importance of the ambient medium.

See also NASA/ADS.


Energetic Phenomena on the Sun

Nat Gopalswamy
KODAI SCHOOL ON SOLAR PHYSICS, AIP Conference Proceedings, Volume 919, pp. 275-313, 2007.

Abstract

Solar flares, coronal mass ejections (CMEs), solar energetic particles (SEPs), and fast solar wind represent the energetic phenomena on the Sun. Flares and CMEs originate from closed magnetic field structures on the Sun typically found in active regions and quiescent filament regions. On the other hand, fast solar wind originates from open field regions on the Sun, identified as coronal holes. Energetic particles are associated with flares, CMEs, and fast solar wind, but the ones associated with CMEs are the most intense. The energetic phenomena have important consequences in the heliosphere and contribute significantly to adverse space weather. This paper provides an over view of the energetic phenomena on the Sun including their origin interplanetary propagation and space weather consequences.

See also NASA/ADS.


Geoeffectiveness of halo coronal mass ejections

N. Gopalswamy, S. Yashiro, and S. Akiyama
JGR Space Physics, Vol. 112, A06112, doi:10.1029/2006JA012149, 2007

Abstract

We studied the geoeffectiveness, speed, solar source, and flare association of a set of 378 halo coronal mass ejections (CMEs) of cycle 23 (1996-2005, inclusive). We compiled the minimum Dst values occurring within 1-5 days after the CME onset. We compared the distributions of such Dst values for the following subsets of halo CMEs: disk halos (within 45 deg from disk center), limb halos (beyond 45 degrees but within 90 deg from disk center), and backside halo CMEs. On the average, the disk halos are followed by intense storms, limb halos are followed by moderate storms, and backside halos are not followed by significant storms. The Dst distribution for a random sample is nearly identical to the case of backside halos. We found that ~71% of all frontside halos are geoeffective, supporting the high rate of geoeffectiveness of halo CMEs. A larger fraction of disk halos were geoeffective. The geoeffectiveness rate had prominent dips in 1999 and 2002 (the beginning and end years of the solar maximum phase). The number of geoeffective halos shows a triple peak similar to the number of intense geomagnetic storms. Intense storms generally were due to disk halos and the few intense storms from limb halos occurred only in the maximum and declining phases. Most intense storms occurred when there were successive CMEs. The difference in flare sizes among geoeffective and non-geoeffective halos is not significant. The non-geoeffective CMEs are generally slower and have more easterly or limbward solar sources compared to the geoeffective ones, source location and speed are the most important parameters for geoeffectiveness.

See also NASA/ADS.


The CME-productivity associated with flares from two active regions

S.Akiyama, S. Yashiro, and N. Gopalswamy
Advances in Space Research, Vol 39, Issue 9, P. 1467, 2007

Abstract

We report on two flare-productive adjacent active regions (ARs), with different levels of coronal mass ejection (CME) association. AR 10039 and AR 10044 produced strong X-ray flares during their disk passages. We examined the CME association rate of X-ray flares and found it to be different between the two ARs. AR 10039 was CME-rich with 72% association with flares, while AR 10044 was CME-poor with an association rate of only 14%. CMEs from the CME-rich AR were faster and wider than the ones from the CME-poor AR. The flare activity of AR 10044 was temporally concentrated over a short interval and spatially localized over a compact area between the major sun spots. We suggest that different pre-eruption evolution and magnetic configuration in the two regions might have contributed to the difference between the two ARs.

A preprint of this paper can be downloaded as a preprint.


2006

PROPERTIES OF INTERPLANETARY CORONAL MASS EJECTIONS

Nat Gopalswamy
Space Science Reviews, Vol. 124, p145, 2006, DOI: 10.1007/s11214-006-9102-1

Abstract

Interplanetary coronal mass ejections (ICMEs) originating from closed field regions on the Sun are the most energetic phenomenon in the heliosphere. They cause intense geomagnetic storms and drive fast mode shocks that accelerate charged particles. ICMEs are the interplanetary manifestations of CMEs typically remote-sensed by coronagraphs. This paper summarizes the observational properties of ICMEs with reference to the ordinary solar wind and the progenitor CMEs.

See also NASA/ADS.


Radio Observations of Solar Eruptions

N. Gopalswamy
in Proceedings of Nobeyama Symposium 2004, pp. 81-94, 2006

Abstract

Coronal mass ejections (CMEs) are composed of multithermal plasmas, which make them produce different radio signatures at different wavelengths. The prominence core of CMEs are of the lowest temperature and hence optically thick at microwave frequencies and hence are readily observed. The Nobeyama Radioheliograph has exploited this fact and observed a large number of prominence eruptions over most of solar cycle 23 and parts of cycle 22. This paper reviews recent studies on prominence eruptions and their contributions for understanding the CME phenomenon. In particular, the following issues are discussed: (i) the statistical and physical relationship between CMEs and the radio prominence eruptions, and how this relationship manifests as a function of the solar cycle; (ii) The asymmetry of prominence eruptions between northern and southern hemispheres; (iii) the relationship between prominence eruptions and CME cores; (iv) the implications of the cessation of high-latitude PEs before the reversal of the global solar magnetic field, and (v) the implications of the high-latitude PEs and CMEs for the modulation of galactic cosmic rays. Finally, the importance of the Nobeyama Radioheliograph data to future missions such as STEREO and Solar-B are discussed.

See also NASA/ADS.


Properties and geoeffctiveness of halo coronal mass ejections

G. Michalek, N. Gopalswamy, A. Lara, and S. Yashiro
Space Weather, Volume 4, Issue 10, CiteID S100003, 2006

Abstract

Halo coronal mass ejections (HCMEs) originating from regions close to the center of the Sun are likely to be geoeffective. Assuming that the shape of HCMEs is a cone and they propagate with constant angular widths and velocities, at least in their early phase, we have developed a technique (Michalek et al. 2003) which allowed us to obtain the space speed, width and source location. We apply this technique to obtain the parameters of all full HCMEs observed by the Solar and Heliospheric Observatory (SOHO) mission's Large Angle and Spectrometric Coronagraph (LASCO) experiment until the end of 2002. Using this data we examine which parameters determine the geoeffectiveness of HCMEs. We show that in the considered period of time only fast halo CMEs (with the space velocities higher than 1000km/s and originating from the western hemisphere close to the solar center could cause the severe geomagnetic storms. We illustrate how the HCME parameters can be used for space weather forecast. It is also demonstrated that the strength of a geomagnetic storm does not depend on the determined width of HCMEs. This means that HCMEs do not have to be very large to cause major geomagnetic storms.

A preprint of this paper can be downloaded from arXiv.


An Asymmetric Cone Model for Halo Coronal Mass Ejections

G. Michalek
Solar Physics, Volumen 237, Issue1, pp.101-118, 2006

Abstract

Due to projection effects, coronagraphic observations cannot uniquely determine parameters relevant to the geoeffectiveness of CMEs, such as the true propagation speed, width, or source location. The Cone Model for Coronal Mass Ejections (CMEs) has been studied in this respect and it could be used to obtain these parameters. There are evidences that some CMEs initiate from a flux-rope topology. It seems that these CMEs should be elongated along the flux-rope axis and the cross section of the cone base should be rather elliptical than circular. In the present paper we applied an asymmetric cone model to get the real space parameters of frontsided halo CMEs (HCMEs) recorded by SOHO/LASCO coronagraphs in 2002. The cone model parameters are generated through a fitting procedure to the projected speeds measured at different position angles on the plane of the sky. We consider models with the apex of the cone located at the center and surface of the Sun. The results are compared to the standard symmetric cone model.

A preprint of this paper can be downloaded from arXiv.


Consequences of Coronal Mass Ejections in the Heliosphere

N. Gopalswamy
Sun and Geosphere (ISSN: 1819-0839), 1(2), 5-12, 2006

Abstract

Coronal mass ejections (CMEs) are the most energetic events in the heliosphere. They carry large amounts of coronal magnetic fields and plasma with them and drive large-scale interplanetary shocks. The CMEs and shock have significant consequences at various locations in the heliosphere, including the production of intense geomagnetic storms and large energetic particle events. CMEs form merged interaction regions in the heliosphere, which act as magnetic barriers for the galactic cosmic rays entering the heliosphere. After a brief summary of the observed properties of CMEs at the Sun, I discuss the properties of the interplanetary CMEs (ICMEs) and their connection to shocks, radio bursts, solar energetic particles and the modulation of galactic cosmic rays.

See also NASA/ADS.


Solar Influence on the Heliosphere and Earth's Environment: Recent Progress and Prospects by N. Gopalswamy and A. Battacharyya (Editors), Quest Publications, Mumbai, 453 pages, hardbound, 2006, ISBN 81-87099-40-2

Preface

The mission of the International Living with a Star (ILWS) program is to stimulate, strengthen, and coordinate space research to understand the governing processes of the connected Sun-Earth System as an integrated entity. Accomplishing this mission involves: (i) study of the Sun-Earth connected system and the effects which influence life and society, (ii) collaboration among potential partners in solar-terrestrial space missions, (iii) synergistic coordination of international research in solar-terrestrial studies, including all relevant data sources as well as theory and modeling, and (iv) effective and user-driven access to all data, results, and value-added products.

The ILWS program can be thought of as the culmination of various international collaborative efforts starting from the Apollo-Soyuz joint project undertaken in 1975 between the United States and the former Soviet Union. Some recent international collaborative missions such as Ulysses, Yohkoh, SOHO, ACE, Hinode, and STEREO have demonstrated the benefits of collaboration for the world scientific community. Currently the ILWS program has over 20 member agencies cooperating in the space missions and science activities. It is expected that between now and the year 2015, more than twenty new science missions of international cooperation will be flown to investigate various domains of importance to ILWS program and the physical processes that link them.

The 2006 ILWS workshop on Solar Influence on the Heliosphere and Earth's Environment: Recent Progress and Prospects is an effort to bring the international space weather and ILWS communities together to address the most critical research problems of solar variability and its impact on the human society. This workshop provided an opportunity to review the current status of our understanding of solar influence on the heliospace, identify promising new lines of research, and provide a venue to identify prospective paths for international cooperation. The scientific program of the workshop was inline with all the objectives of the ILWS program.

The workshop deliberations had programmatic sessions, plenary sessions, working group sessions, and poster sessions. The plenary sessions consisted of review papers that highlighted the current issues in understanding the physical processes from the Sun to the edge of the solar system. The working group sessions had more intense discussions on specific topics based on traditional disciplines of solarterrestrial physics: (1) solar-heliosphere, (2) magnetosphere, and (3) ionosphere-thermospheremesosphere. Plenary sessions also included discussions on the cross-disciplinary aspects of issues raised in the working groups. Finally, there was a panel discussion on future collaborations among members of the scientific community from different countries.

The papers collected in this volume represent a subset of papers presented in the workshop that serve as a record of the workshop proceedings. The review papers will be useful to researchers on solar-terrestrial physics to gain a quick update of the current issues. The contributed papers present original research work. Summaries of the three working group sessions are also included in the front material to serve as introduction to the papers included in this volume. Also included is a summary of the panel discussion on future collaborations.

Papers dealing with all aspects of solar and heliospheric physics are represented: helioseismology, solar atmosphere, solar eruptions (coronal mass ejections and flares), solar irradiance, solar wind, and heliospheric impact. Papers on theory, modeling, and future space missions are also included. Papers in the section devoted to the magnetosphere cover a range of topics including solar wind coupling to the magnetosphere, radiation belts and energetic particles, waves and fluctuations and their effects, stormsubstorm relationship, modeling and prediction, and new missions. In the section dealing with ionosphere, thermosphere, and mesosphere, the papers discuss a number of issues related to effects of solar variability on this region of geospace, observed using satellite and ground-based data including ground magnetometer observations, radio beacon studies of equatorial spread F, and modeling of some of these effects. Radar observations of the mesosphere-lower thermosphere region and a future mission to study the coupling of thunderstorm processes to this region, the ionosphere, and magnetosphere, are also described.

This volume would not have seen the light without the untiring help provided by V. Reddy, S. Singh, A. Kakad, B. Veenadhari, B. I. Panchal (all from Indian Institute of Geomagnetism) and S. Petty (Catholic University of America). Financial support from the Department of Science and Technology (India) and NASA s Living with a Star program enabled many key scientists participate in the workshop, which was organized by the Indian Institute of Geomagnetism. Specials thanks are due to J. Rumburg for creating the conference graphics and web site. Finally the editors are very grateful to the excellent conference arrangements made by the local organizing committee. The scientific organizing committee was responsible for selecting the best set of talks and posters, many of which are printed here.

Nat Gopalswamy
NASA Goddard Space Flight Center, Geenbelt, Maryland

Archana Bhattacharyya
Indian Institute of Geomagnetism, Navi Mumbai India

The book is available online.


Coronal mass ejections and space weather due to extreme events

N. Gopalswamy, S. Yashiro and S. Akiyama
in "Solar Influence on the Heliosphere and Earth's Environment: Recent Progress and Prospects",
ed. N. Gopalswamy and A. Battacharyya, Quest Publications, Mumbai, p. 79, 2006.

Abstract

This paper summarizes the extreme solar activity and its space weather implications during the declining phase of the solar cycle 23: October-November 2003 (AR 486), November 2004 (AR 696), January 2005 (AR 720), and September 2005 (AR 808). We have compiled and compared the properties of eruptions and the underlying active regions. All these are super active regions, but the flare and CME productivity varied significantly. While the CMEs from all the regions kept the level of solar energetic particles (SEPs) at storm level for several days, their geoeffectiveness (the ability to produce geomagnetic storms) was significantly different, probably due to the location of the eruptions on the Sun.

A preprint of this paper can be downloaded as a pdf file.


Coronal mass ejections and space weather

D. F. Webb and N. Gopalswamy
in "Solar Influence on the Heliosphere and Earth's Environment: Recent Progress and Prospects",
ed. N. Gopalswamy and A. Battacharyya, Quest Publications, Mumbai, p. 71, 2006.

Abstract

Coronal mass ejections (CMEs) are a key feature of coronal and interplanetary (IP) dynamics. Major CMEs inject large amounts of mass and magnetic fields into the heliosphere and, when aimed Earthward, can cause major geomagnetic storms and drive IP shocks, a key source of solar energetic particles. Studies over this solar cycle using the excellent data sets from the SOHO, TRACE, Yohkoh, Wind, ACE and other spacecraft and ground-based instruments have improved our knowledge of the origins and early development of CMEs at the Sun and how they affect space weather at Earth. A new heliospheric experiment, the Solar Mass Ejection Imager, has completed 3 years in orbit and has obtained results on the propagation of CMEs through the inner heliosphere and their geoeffectiveness. We review key coronal properties of CMEs, their source regions, their manifestations in the solar wind, and their geoeffectiveness. Halo-like CMEs are of special interest for space weather because they suggest the launch of a geoeffective disturbance toward Earth. However, not all halo CMEs are equally geoeffective and this relationship varies over the solar cycle. Although CMEs are involved with the largest storms at all phases of the cycle, recurrent features such as interaction regions and high speed wind streams can also be geoeffective.

A preprint of this paper can be downloaded as a pdf file.


Preparing for the International Heliophysical Year (IHY) 2007

J. M. Davila, N. Gopalswamy and B. J. Thompson
in "Solar Influence on the Heliosphere and Earth's Environment: Recent Progress and Prospects",
ed. N. Gopalswamy and A. Battacharyya, Quest Publications, Mumbai, p. 231, 2006.

Abstract

The International Geophysical Year (IGY) of 1957, a broad-based and all-encompassing effort to push the frontiers of geophysics, resulted in a tremendous increase of knowledge in space physics, Sun-Earth Connection, planetary science and the heliosphere in general. Now, 50 years later, we have the unique opportunity to advance our knowledge of the global heliosphere and its interaction with planetary bodies and the interstellar medium through the International Heliophysical Year (IHY) in 2007. This will be an international effort, which will raise public awareness of space physics.

A preprint of this paper can be downloaded as a pdf file.


Solar Eruptions and Energetic Particles, ed. N. Gopalswamy, R. Mewaldt, and J. Torsti,
Geophysical Monograph Series 165, American Geophysical Union, p. ix, 2006, doi: 10.1029/165GM01

Preface

Research over the last three decades identifies coronal mass ejections (CMEs) as the most energetic events in the heliosphere. Although studies of solar energetic particle (SEP) events and nonthermal radio bursts have a longer history, the close connection between CMEs and energetic particles has become much clearer thanks to the large armada of spacecraft observing these phenomena since the mid-1990s. Indeed, understanding the most violent forms of solar eruptions -- CMEs, flares, and SEPs -- is of fundamental importance to the physics involved and our ability to predict and mitigate disruptive space weather episodes. Questions, of course, remain: We do not fully understand how CMEs and SEPs are accelerated but we do know that they affect space weather in several significant ways. The magnetized plasma of CMEs impacts Earth's magnetosphere, causing large geomagnetic storms. Energetic CMEs also drive shocks that accelerate electrons (observed as type II radio bursts) and ions (detected by spaceborne instruments). SEPs ionize the upper atmosphere, disrupting communications, driving atmospheric chemistry, while presenting a radiation hazard to humans and hardware in space.

This volume reviews extensive observations of solar eruptions and SEPs by instruments on board a number of spacecraft, including the Solar and Heliospheric Observatory (SOHO), Wind, the Advanced Composition Explorer (ACE), the Ramaty High Energy Solar Spectroscopic Imager (RHESSI), and the Transition Region and Coronal Explorer (TRACE). Highly sensitive coronagraphs on board SOHO image CMEs with unprecedented sensitivity. Several thousand CMEs have been observed, measured and cataloged for the current solar cycle, but only about 1% of these are associated with SEPs. Radio instruments on the Wind spacecraft obtain signatures of solar energetic electrons injected into the heliosphere within minutes of their release near the Sun and also track MHD shocks driven by CMEs. A majority of space instruments detect SEPs in situ and measure their elemental, ionic charge state, and isotopic compositions. Thus, it has become possible to link the evolution of SEP events to CME-driven shocks as they propagate from the Sun to geospace and beyond. In early 2002, RHESSI began complementing these observations with high-resolution imaging of x-rays and gamma rays from flares associated with these events. These multi-spacecraft, multi-instrument and multi-wavelength observations have raised more pointed questions about the origin, acceleration, and interplanetary propagation of SEPs. This volume records advances made in the understanding of solar eruptions with significant consequences in the heliosphere.

The volume is organized into five topical areas, with an introductory review of the early development and current state of CME and energetic particle studies. Topical areas include: CMEs, SEPs, connection to flares, associated phenomena, and space weather. In-depth reviews on solar eruptions and energetic particles also contain observational studies, discussion of theoretical developments, and modeling results. The papers on associated phenomena deal with flares, type II radio bursts, and shock waves. After considering the interplanetary propagation of CMEs and energetic particles, space weather implications are discussed, including the arrival of energetic particles at geospace and their impact on Earth's radiation belts. The review papers cover all important aspects of CMEs and energetic particles making the volume largely self-contained.

Most of the papers in this volume were presented at an AGU Chapman Conference, entitled "Solar Energetic Plasmas and Particles," held at the University of Turku, Finland, August 2-6, 2004. Several additional papers were solicited to make the volume as complete a survey of the subject as possible. Two experts provided peer review for each paper. The editors appreciate the constructive and timely reviews by many members of the international space weather, and Living with a Star, communities that have greatly enhanced the quality of this volume. Finally, the editors are very grateful for the excellent conference arrangements made by the local organizing committee headed by Eino Valtonen from the University of Turku.

Nat Gopalswamy
NASA Goddard Space Flight Center, Greenbelt, Maryland

Richard Mewaldt
California Institute of Technology, Pasadena, California

Jarmo Torsti
University of Turku, Turku, Finland


Solar Eruptions and Energetic Particles: An Introduction

N. Gopalswamy, R. Mewaldt, and J. Torsti
in Solar Eruptions and Energetic Particles, ed. N. Gopalswamy, R. Mewaldt, and J. Torsti,
Geophysical Monograph Series 165, American Geophysical Union, pp. 1-5, 2006, doi: 10.1029/165GM012

Abstract

This introductory article highlights current issues concerning two related phenomena involving mass emission from the sun: solar eruptions and solar energetic particles. A brief outline of the chapters is provided indicating how the current issues are addressed in the monograph. The sections in this introduction roughly group the chapters dealing with coronal mass ejections (CMEs), solar energetic particles (SEPs), shocks, and space weather. The concluding remarks include a brief summary of outstanding issues that drive current and future research on CMEs and SEPs.


Coronal Observations of CMEs

Schwenn, R.; Raymond, J. C.; Alexander, D.; Ciaravella, A.; Gopalswamy, N.; Howard, R.; Hudson, H.; Kaufmann, P.; Klassen, A.; Maia, D.; Munoz-Martinez, G.; Pick, M.; Reiner, M.; Srivastava, N.; Tripathi, D.; Vourlidas, A.; Wang, Y.-M.; Zhang, J.
Space Science Reviews, Volume 123, Issue 1-3, pp. 127-176, 2006

Abstract

CMEs have been observed for over 30 years with a wide variety of instruments. It is now possible to derive detailed and quantitative information on CME morphology, velocity, acceleration and mass. Flares associated with CMEs are observed in X-rays, and several different radio signatures are also seen. Optical and UV spectra of CMEs both on the disk and at the limb provide velocities along the line of sight and diagnostics for temperature, density and composition. From the vast quantity of data we attempt to synthesize the current state of knowledge of the properties of CMEs, along with some specific observed characteristics that illuminate the physical processes occurring during CME eruption. These include the common three-part structures of CMEs, which is generally attributed to compressed material at the leading edge, a low-density magnetic bubble and dense prominence gas. Signatures of shock waves are seen, but the location of these shocks relative to the other structures and the occurrence rate at the heights where Solar Energetic Particles are produced remains controversial. The relationships among CMEs, Moreton waves, EIT waves, and EUV dimming are also cloudy. The close connection between CMEs and flares suggests that magnetic reconnection plays an important role in CME eruption and evolution. We discuss the evidence for reconnection in current sheets from white-light, X-ray, radio and UV observations. Finally, we summarize the requirements for future instrumentation that might answer the outstanding questions and the opportunities that new space-based and ground-based observatories will provide in the future.

See also NASA/ADS.


On the Rates of Coronal Mass Ejections: Remote Solar and In Situ Observations

Riley, Pete; Schatzman, C.; Cane, H. V.; Richardson, I. G.; Gopalswamy, N.
The Astrophysical Journal, Volume 647, Issue 1, pp. 648-653, 2006

Abstract

We compare the rates of coronal mass ejections (CMEs) as inferred from remote solar observations and interplanetary CMEs (ICMEs) as inferred from in situ observations at both 1 AU and Ulysses from 1996 through 2004. We also distinguish between those ICMEs that contain a magnetic cloud (MC) and those that do not. While the rates of CMEs and ICMEs track each other well at solar minimum, they diverge significantly in early 1998, during the ascending phase of the solar cycle, with the remote solar observations yielding approximately 20 times more events than are seen at 1 AU. This divergence persists through 2004. A similar divergence occurs between MCs and non-MC ICMEs. We argue that these divergences are due to the birth of midlatitude active regions, which are the sites of a distinct population of CMEs, only partially intercepted by Earth, and we present a simple geometric argument showing that the CME and ICME rates are consistent with one another. We also acknowledge contributions from (1) an increased rate of high-latitude CMEs and (2) focusing effects from the global solar field. While our analysis, coupled with numerical modeling results, generally supports the interpretation that whether one observes a MC within an ICME is sensitive to the trajectory of the spacecraft through the ICME (i.e., an observational selection effect), one result directly contradicts it. Specifically, we find no systematic offset between the latitudinal origin of ICMEs that contain MCs at 1 AU in the ecliptic plane and that of those that do not.

See also NASA/ADS.


Are halo coronal mass ejections special events?

Lara, Alejandro; Gopalswamy, Nat; Xie, Hong; Mendoza-Torres, Eduardo; Perez-Eriquez, Roman; Michalek, Gregory
Journal of Geophysical Research, Volume 111, Issue A6, CiteID A06107, 2006

Abstract

Abstract We revisited the properties of wide coronal mass ejections (CMEs) called halo CMEs. Using the large LASCO/SOHO CMEs data set, from 1996 to 2004, we examined the statistical properties of (partial and full) halo CMEs and compare with the same properties of ``normal'' width (lower than 120 deg) CMEs. We found that halo CMEs have different properties than ``normal'' CMEs, which cannot be explained merely by the current geometric interpretation that they are seen as halos because they are traveling in the Sun Earth direction. We found that the CME width distribution is formed by, at least, three different populations: Two gaussians: a narrow and a medium distribution centered at ~17 deg and ~38 deg, respectively; the narrow population most likely corresponds to the ``true'' observed widths, whereas the medium width population is the product of projection effects. The third distribution corresponds to wider CMEs (80 deg < W < 210 deg) which behaves as a power law. Partial and full halo CMEs wider than these do not follow any particular distribution. This lack of regularity may be due to the small number of such events. In particular, we found (and test by a statistical approach) that the number of observed full halo CMEs is lower than expected. The CME speed follows a log-normal distribution, except for the very low speed CME population, which follows a gaussian distribution centered at ~100 km/s and is probably due to projection effects. When the CMEs are divided by width into nonhalo, partial halo, and full halo, we found that the peaks of the distributions are shifted toward higher speeds, ~300, ~400 and ~600 km/s for nonhalo, partial halo, and full halo CMEs, respectively. This confirms that halo CMEs tend to be high speed CMEs. The acceleration of full halo CMEs tends to be more negative compared with nonhalo and partial halo CMEs. We introduce a new observational CME parameter: The final observed distance (FOD), i.e., the highest point within the coronograph field of view where a CME can be distinguished from the background. In other words, the highest CME altitude measured. The FOD for nonhalo CMEs decreases exponentially from ~5 to ~30 RS in the LASCO field of view. On the other hand, the FOD of halo CMEs increase with distance. This means that it is more likely to see halo CMEs at large distances (from the Sun) than nonhalo CMEs. These halo CME properties may be explained if the white light wide enhancements (or halo) seen by coronographs correspond to a combination of an expanding (shock) wave which disturbs and/or compresses the ambient material and the CME material itself.

See also NASA/ADS.


Solar Sources of Impulsive Solar Energetic Particle Events and Their Magnetic Field Connection to the Earth

Nitta, Nariaki V.; Reames, Donald V.; DeRosa, Marc L.; Liu, Yang; Yashiro, Seiji; Gopalswamy, Natchimuthuk
The Astrophysical Journal, Volume 650, Issue 1, pp. 438-450, 2006

Abstract

This paper investigates the solar origin of impulsive solar energetic particle (SEP) events, often referred to as 3He-rich flares, by attempting to locate the source regions of 117 events as observed at ~2-3 MeV/amu. Given large uncertainties as to when ions at these energies were injected, we use type III radio bursts that occur within a 5 hr time window preceding the observed ion onset, and search in EUV and X-ray full-disk images for brightenings around the times of the type III bursts. In this way we find the solar sources in 69 events. High cadence EUV images often reveal a jet in the source region shortly after the type III burst. We also study magnetic field connections between the Earth and the solar sources of impulsive SEP events as identified above, combining the potential field source surface (PFSS) model for the coronal field and the Parker spiral for the interplanetary magnetic field. We find open field lines in and around ~80% of the source regions. But only in ~40% of the cases, can we find field lines that are both close to the source region at the photosphere and to the Parker spiral coordinates at the source surface, suggesting challenges in understanding the Sun-Earth magnetic field with observations available at present and in near future.

See also NASA/ADS.


Composition and magnetic structure of interplanetary coronal mass ejections at 1 AU

Aguilar-Rodriguez, E.; Blanco-Cano, X.; Gopalswamy, N.
Advances in Space Research, Volume 38, Issue 3, p. 522-527, 2006

Abstract

We study the magnetic structure and charge state ratio of interplanetary coronal mass ejections (ICMEs) observed by ACE and Wind spacecraft. Measurements of abundances and charge state ratio of heavy ions (e.g. O7+/O6+, C6+/C5+, and Mg10+/O6+) in the plasma as well as magnetic field structure are important tracers for physical conditions and processes in the source regions of ICMEs. We used ion composition (from ACE), plasma (from Wind) and magnetic field (from Wind and ACE) data from 1998 to 2002. Using the low proton temperature criterion, a common plasma signature of ICMEs, we identified 154 events which include magnetic clouds, non-cloud ejecta and complex ICMEs. The latter one refers to compound events resulting from the overtaking of successive ICMEs which can include both magnetic clouds and non-cloud ejecta. We find that there is a close relationship between the increase in the charge state ionization factor and the magnetic structure of ICMEs. Events with magnetic cloud topology show higher QandQ charge state ratios than those with non-magnetic cloud structure. However, both magnetic cloud and non-cloud events show an increase in these ratios when compared with the ambient solar wind. In contrast, perhaps due to instrumental effects, the charge state ratio Q for all events does not show a real enhancement when compared with the ambient solar wind. The difference in ionization states between non-cloud ejecta and magnetic clouds is more pronounced in fast solar wind than when events are embedded in slow wind.

See also NASA/ADS.


Relationships Among Magnetic Clouds, CMES, and Geomagnetic Storms

Wu, C. C.; Lepping, R. P.; Gopalswamy, N.
Solar Physics, Volume 239, Issue 1-2, pp. 449-460, 2006.

Abstract

During solar cycle 23, 82 interplanetary magnetic clouds (MCs) were identified by the Magnetic Field Investigation (MFI) team using Wind (1995 - 2003) solar wind plasma and magnetic field data from solar minimum through the maximum of cycle 23. The average occurrence rate is 9.5 MCs per year for the overall period. It is found that some of the anomalies in the frequency of occurrence were during the early part of solar cycle 23: (i) only four MCs were observed in 1999, and (ii) an unusually large number of MCs (17 events) were observed in 1997, just after solar minimum. We also discuss the relationship between MCs, coronal mass ejections (CMEs), and geomagnetic storms. During the period 1996 - 2003, almost 8000 CMEs were observed by SOHO-LASCO. The occurrence frequency of MCs appears to be related neither to the occurrence of CMEs as observed by SOHO LASCO nor to the sunspot number. When we included "magnetic cloud-like structures" (MCLs, defined by Lepping, Wu, and Berdichevsky, 2005), we found that the occurrence of the joint set (MCs + MCLs) is correlated with both sunspot number and the occurrence rate of CMEs. The average duration of the MCL structures is ~40% shorter than that of the MCs. The MCs are typically more geoeffective than the MCLs, because the average southward field component is generally stronger and longer lasting in MCs than in MCLs. In addition, most severe storms caused by MCs/MCLs with Dstmin ≤ 100 nT occurred in the active solar period.

See also NASA/ADS.


Highlights of the October-November 2003 Extreme Events

N. Gopalswamy
in "Solar Extreme Events: Fundamental Science and Applied Aspects" ed. A. Chilingarian and G. Karapetyan, Cosmic Ray Division, Alikhanyan Physics Institute, Yerevan, pp. 20-24, 2006

There was a high concentration of coronal mass ejections (CMEs), X-class soft X-ray flares, solar energetic particle (SEP) events, and interplanetary shocks observed during the episode the late October and early November 2003 period. The CMEs were very energetic and the consequences were also unusually intense. These extreme properties were commensurate with the size and energy of the associated active regions. This study suggests that the speed of CMEs may not be much higher than ~3000 km/s, consistent with the large free energy available in the associated active regions. The observations indicate that the CMEs may not have speeds much higher than ~ 3000 km/s implying that the Sun-Earth travel times of CME-driven shocks may not be less than ~0.5 day. Some of the CMEs were both geoeffective and SEPeffective, which are the most important from a space weather point of view.

A preprint of this paper can be downloaded as a pdf file.


Different Power-law Indices in the Frequency Distributions of Flares with and without Coronal Mass Ejections

S. Yashiro, S. Akiyama, N. Gopalswamy, and R. A. Howard
The Astrophysical Journal, 650, L143, 2006.

Abstract

We investigated the frequency distributions of flares with and without coronal mass ejections (CMEs) as a function of flare parameters (peak flux, fluence, and duration of soft X-ray flares). We used CMEs observed by the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO) mission and soft X-ray flares (C3.2 and above) observed by the GOES satellites during 1996 to 2005. We found that the distributions obey a power-law of the form: dN/dX∝X, where X is a flare parameter and dN is the number of events recorded within the interval [X, X+dX]. For the flares with (without) CMEs, we obtained the power-law index α=1.98±0.05 (α=2.52±0.03) for the peak flux, α=1.79±0.05 (α=2.47±0.11) for the fluence, and α=2.49±0.11 (α=3.22±0.15) for the duration. The power-law indices for flares without CMEs are steeper than those for flares with CMEs. The larger power-law index for flares without CMEs supports the possibility that nanoflares contribute to coronal heating.

A preprint of this paper can be downloaded from NASA/ADS.


THE PRE-CME SUN

N. Gopalswamy, Z. Mikic, D. Maia, D. Alexander, H. Cremades, P. Kaufmann, D. Tripathi, and Y.-M. Wang
Space Science Reviews, Volume 123, Issue 1-3, pp. 303-339, 2006

Abstract

The coronal mass ejection (CME) phenomenon occurs in closed magnetic field regions on the Sun such as active regions, filament regions, transequatorial interconnection regions, and complexes involving a combination of these. This chapter describes the current knowledge on these closed field structures and how they lead to CMEs. After describing the specific magnetic structures observed in the CME source region, we compare the substructures of CMEs to what is observed before eruption. Evolution of the closed magnetic structures in response to various photospheric motions over different time scales (convection, differential rotation, meridional circulation) somehow leads to the eruption. We describe this pre-eruption evolution and attempt to link them to the observed features of CMEs. Small-scale energetic signatures in the form of electron acceleration (signified by nonthermal radio bursts at metric wavelengths) and plasma heating (observed as compact soft X-ray brightening) may be indicative of impending CMEs. We survey these pre-eruptive energy releases using observations taken before and during the eruption of several CMEs. Finally, we discuss how the observations can be converted into useful inputs to numerical models that can describe the CME initiation.

See also NASA/ADS.


Comment on Interplanetary shocks unconnected with earthbound coronal mass ejections by T. A. Howard and S. J. Tappin

Gopalswamy, Nat; Akiyama, Sachiko; Yashiro, Seiji; Kasper, J.
Geophys. Res. Lett., Vol. 33, No. 11, L11108

Abstract

Recently, Howard and Tappin [2005] (hereinafter referred to as HT) reported on a set of 7 interplanetary (IP) shocks, apparently not connected with any detectable coronal mass ejection (CME) activity along the Sun-Earth line and concluded that there was no evidence to associate 6 of them with corotating interaction regions (CIRs); they were uncertain about one event. Based on these results, HT put forth a proposal that the 6 shocks were associated with "erupting magnetic structures" or EMSs and that EMSs rather than CIRs are the dominant cause of IP shocks that cannot be associated with halo CMEs. Our analysis of these events does not agree with these conclusions due the following reasons: (1) the Solar and Heliospheric Observatory (SOHO) mission had a data gap for one event , and (23 October 1998), so the CME association could not be checked; (2) the 18 May 1999 and 23 December 2001 shocks were likely CIR-related; (3) the remaining 4 shocks were CME-related, two (7 April 1998 and 9 November 2002) reported in the published literature [Manoharan et al., 2004] and the other two (both on 23 August 1999) were associated with two successive CMEs from the same region ejected off the Sun-earth line.

See also NASA/ADS.


The Solar Imaging Radio Array: Space-Based Imaging of Solar, Heliospheric, Magnetospheric, and Astrophysics Sources at Frequencies below the Ionospheric Cutoff

MacDowall, R.J.; Gopalswamy, N.; Kaiser, M.L.; Bale, S.D.; Demaio, L.D.; Hewitt; J.N.; Kasper, J.C.; Lazarus, A.J.; Howard, R.E.; Jones, D.L.; Reiner, M.J.; Weiler, K.W.
in From Clark Lake to the Long Wavelength Array: Bill Erickson's Radio Science, ASP Conference Series, Vol. 345, ed.: Kassim, Namir E.; Perez, Mario R.; Junor, William; Henning, Patricia A., p. 476, 2006

Abstract

Solar Imaging Radio Array (SIRA) is a mission concept for spacebased, interferometric imaging of solar and interplanetary radio emission at frequencies below the Earth's ionospheric cutoff. Observing in a frequency range of ~30 kHz to 15 MHz, SIRA will observe the radio emission from shocks driven by fast coronal mass ejections (CMEs). The radio emissions permit tracking the leading boundaries of CMEs from ~2 Rs to 1 AU. When a CME impacts Earth's magnetosphere, the dynamic response will be imaged in the light of magnetospheric radio emissions, such as auroral kilometric radiation (AKR), scattered on magnetospheric density gradients. The near-term possibility for a SIRA mission is based on a NASA MIDEX-class mission, consisting of a single constellation of ~16 microsats located quasi-randomly on a spherical shell of ~10 km diameter. Such a mission is the logical next step in space-based solar radio observations, as well as oRering a unique space weather prediction capability for the NASA Exploration Initiative. SIRA will also serve a valuable role as a pathfinder for more complex constellation and interferometry missions.

A preprint of this paper can be downloaded from NASA/ADS.


Solar wind speed within 20 RS of the Sun estimated from limb coronal mass ejections

Tomoko Nakagawa, Nat Gopalswamy, and Seiji Yashiro
J. Geophys. Res., 111, A01108, doi:10.1029/2005JA011249, 2006

Abstract

An estimation of the solar wind speed in the vicinity of the Sun is carried out using the initial speed and acceleration of coronal mass ejections (CMEs) that appeared close to the solar limb. A linear relationship was found between the initial acceleration and the speed of the limb CMEs. It appears that a dragging force is acting on the CMEs, depending on the speed difference between the CMEs and the ambient plasma. The ambient solar wind speed within 20 solar radii estimated from low-latitude CMEs during 1998-2003 ranged from 100 to 700 km/s, while the solar wind speed measured at 1 AU ranged from 300 to 700 km/s. The estimated solar wind speeds in the vicinity of the Sun sometimes agreed with the simultaneous in situ measurements at 1 AU, but in other periods they were slower than the speeds measured at 1 AU. It is suggested that most of the time the low-latitude solar wind completes accelerating within 20 solar radii, but occasionally additional acceleration is present beyond 20 solar radii.

See also pdf file.


Coronal Mass Ejections and Type II Radio Bursts

N. Gopalswamy
in "Solar Eruptions and Energetic Particles", Geophysical Monograph 165, ed. N. Gopalswamy, R. Mewaldt, and J. Torsti, pp. 207-220, doi:10.1029/165GM20, 2006.

Abstract

The simultaneous availability of white light data on CMEs from the Solar and Heliospheric Observatory (SOHO) and radio data on shock waves from the Radio and Plasma Wave experiment on board the Wind spacecraft over the past decade have helped in making rapid pro-gress in understanding the CME-driven shocks. I review some recent de-velopments in the type II - CME relationship, focusing on the properties of CMEs as shock drivers and those of the medium supporting shock propagation. I also discuss the solar cycle variation of the type II bursts in comparison with other eruptive phenomena such as CMEs, flares, large solar energetic particle events, and shocks detected in situ. The hierarchi-cal relationship found between the CME kinetic energy and wavelength range of type II radio bursts, non-existence of CMEless type II bursts, and the explanation of type II burst properties in terms of shock propagation with a realistic profile of the fast mode speed suggest that the underlying shocks are driven by CMEs, irrespective of the wavelength domain. Such a unified approach provides an elegant understanding of the entire type II phenomenon (coronal and interplanetary). The blast wave scenario re-mains an alternative hypothesis for type II bursts only over a small spatial domain (within one solar radius above the solar surface) that is not acces-sible to in situ observation. Therefore the existence of blast waves cannot be directly confirmed. CMEs, on the other hand, can be remote sensed from this domain.

A preprint of this paper can be downloaded as a pdf file.


Coronal Mass Ejections of cycle 23

N. Gopalswamy
J. Astrophys. Astron., 27, 243-254, 2006.

Abstract

I summarize the statistical, physical, and morphological properties of coronal mass ejections (CMEs) of solar cycle 23, as observed by the Solar and Heliospheric Observatory (SOHO) mission. The SOHO data is by far the most extensive data, which made it possible to fully establish the properties of CMEs as a phenomenon of utmost importance to Sun-Eath connection as well as to the heliosphere. I also discuss various subsets of CMEs that are of primary importance for impact on Earth.

A preprint of this paper can be downloaded as a pdf file.


2005

Major Scientific Results from SOHO on Coronal Mass Ejections

N. Gopalswamy, B. Fleck and J. B. Gurman
Proceedings of Asia Pacific Regional Conference of IAA "BRINGING SPACE BENEFITS TO THE ASIA REGION" Editors: Mukund RAO & RLN MURTHY, ATRONAUTICAL SOCIETY OF INDIA, BANGALORE, INDIA, 2005

Abstract

Major scientific results related to coronal mass ejections (CMEs) observed by the Solar and heliospheric Observatory (SOHO) mission are discussed. After a brief description of the general properties of CMEs, their relationship to geomagnetic storms, solar energetic particles, and radio bursts is discussed. Also discussed are the CME-driven shocks and their interaction with other CMEs.

A preprint of this paper can be downloaded as a pdf file.


Workshop Highlights Progress in Solar-Heliospheric Physics

N. Gopalswamy
EOS 80, no. 50, 525, 2005


See also NASA/ADS.


Long Lived Geomagnetic Storms and Coronal Mass Ejections

H. Xie, N. Gopalswamy, P.K. Manoharan, A. Lara, S. Yashiro, and S. Lepri
Jornal of Geophysical Research, Vol 111, Issue A1 Cite ID A01103.

Abstract

Coronal mass ejections (CMEs) are major solar events that are known to cause large geomagnetic storms (Dst<-100 nT). Isolated geomagnetic storms typically have a main phase of 3 -12 hours and a recovery phase of around 1 day. However, there are some storms with main and recovery phases exceeding ~ 3 days. We trace the origin of these long-lived geomagnetic storms (LLGMS) to front-side halo CMEs. We studied 37 LLGMS events with Dst < -100 nT and the associated CMEs which occurred during 1998-2002. It is found that LLGMS events are caused by: 1) successive CMEs, accounting for ~64.9 % (24 of 37); 2) single CMEs, accounting for ~21.6 % (8 of 37); and 3) High speed streams (HSS) in corotating interaction regions (CIRs) with no related CME, accounting for ~13.5 % (5 of 37). The long duration of the LLGMS events was found to be due to successive CMEs and HSS events; the high intensity of the LLGMS events was related to the interaction of CMEs with other CMEs and HSS events. We find that the duration of LLGMS is well correlated to the number of participating CMEs (correlation coefficient r = 0.78). We also find that the intensity of LLGMS has a good correlation with the degree of interaction (the number of CMEs interacting with a HSS event or with themselves) (r = 0.67). The role of preconditioning in LLGMS events, where the Dst development occurred in multiple steps in the main and recovery phases, has been investigated. It is found that preconditioning does not affect the main phase of the LLGMS events, while it plays an important role during the recovery phase of the LLGMS events.

A preprint of this paper can be downloaded as a pdf file.


Type II Radio Bursts and Energetic Solar Eruptions

N. Gopalswamy, E. Aguilar-Rodriguez, S. Yashiro, S. Nunes, M. L. Kaiser, and R. A. Howard
JGR, Vol. 110, No. A12, A12S07, doi:10.1029/2005JA011158, 2005

Abstract

We report on a study of type II radio bursts from the Wind/WAVES experiment in conjunction with white-light coronal mass ejection (CME) data from the Solar and Heliospheric Observatory (SOHO). The type II bursts considered here have emission components in all the spectral domains: metric, decameter-hectometric (DH) and kilometric (km), so we refer to them as m-to-km type II bursts. CMEs associated with the m-to-km type II bursts were more energetic than those associated with bursts in any single wavelength regime. CMEs associated with type II bursts confined to the metric domain were more energetic (wider and faster) than the general population of CMEs but less energetic than CMEs associated with DH type II bursts. Thus, the CME kinetic energy seems to organize the life time of the type II bursts. Contrary to previous results, the starting frequency of metric type II bursts with interplanetary counterparts seems to be no different from that of type II bursts without interplanetary counterparts. We also verified this by showing that the average CME height at the onset time of the type II bursts is the same for the two metric populations. The majority (78%) of the m-to-km type were associated with solar energetic particle events. The solar sources of the small fraction of m-to-km type II bursts without SEP association were poorly connected to the observer near Earth. Finally, we found that the m-to-km type II bursts are associated with bigger flares compared to the m-limb type II bursts.

See also NASA/ADS.


Visibility of coronal mass ejections as a function of flare location and intensity

S. Yashiro, N. Gopalswamy, S. Akiyama, G. Michalek, and R. A. Howard
JGR, Vol. 110, No. A12, A12S05, doi:10.1029/2005JA011151, 2005

Abstract

We report the visibility (detection efficiency) of coronal mass ejections (CMEs) of the Large Angle Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO). We collected 1301 X-ray flare events (above C3 level) detected by the GOES satellite, and examined their CME associations using data form LASCO coronagraphs. The CME visibility was examined using the longitudinal variation of CME association of X-ray flares, under the assumption that all CMEs associated with limb flares are detectable by LASCO. Our findings are: (1) the CME association rate clearly increased with X-ray flare size from 20% for C-class flares (between C3 and C9 levels) to 100% for huge flares (above X3 level), (2) all CMEs associated with X-class flares were detected by the LASCO coronagraphs while half (25-67%) of CMEs associated with C-class flares were invisible. We examined the statistical properties of the flare-associated CMEs and compared them by flare size and longitude. CMEs associated with X-class flares were significantly faster (median 1556 km/s) and wider (median 244 deg) than those of CMEs associated with disk C-class flares (432 km/s, 68 deg). We conclude that all fast and wide CMEs are detectable by LASCO, but slow and narrow CMEs may not be visible when the CMEs originate from the disk center.

See also NASA/ADS.


A Universal Characteristic of Type II Radio Bursts

E. Aguilar-Rodriguez, N. Gopalswamy, R. MacDowall, S. Yashiro, and M. L. Kaiser
JGR, Vol. 110, No. A12, A12S08, doi:10.1029/2005JA011171, 2005

Abstract

We present a study on the spectral properties of interplan- etary type II radio bursts observed by the Radio and Plasma Wave (WAVES) experiment on board the Wind spacecraft. We investigated the relative band- width of the type II radio bursts observed by WAVES from 1997 up to 2003. We obtained three sets of events, based on the frequency domain of occur- rence: 109 events in the low frequency domain (30 KHz to 1000 kHz, detected by the RAD1 receiver), 216 events in the high frequency domain (1-14 MHz, observed by the RAD2 receiver), and 73 events that spanned both domains (RAD1 and RAD2). Statistical results show that the average bandwidth-to- frequency ratio (BFR) was 0.28 N' 0.15, 0.26 N' 0.16, and 0.32 N' 0.15 for RAD1, RAD2, and RAD1+RAD2, respectively. We compared our results with those obtained for ISEE-3 type II bursts and found a difference in the average BFR, which seems to be due to a selection effect. The BFR of the WAVES type II bursts is similar to that of metric type II bursts reported in published works. This suggests that the BFR is a universal characteristic, irrespective of the spectral domain. Finally, we also studied the BFR evolution with heliocen- tric distance using white-light observation of the associated coronal mass ejec- tions. We found that the BFR remains roughly constant in the SOHO/LASCO N/eld of view (i.e. from 2.1 to 32 solar radii), while the bandwidth itself de- creases.

See also NASA/ADS.


Coronal Mass Ejections and Ground Level Enhancements

N. Gopalswamy, H. Xie, S. Yashiro, I. Usoskin
in proceeding of 29th International Cosmic Ray Conference, Pune, 2005

Abstract

We study the relation between ground level enhancements (GLEs) and coronal mass ejections (CMEs). The Solar and Heliospheric Observatory (SOHO) spacecraft has observed CMEs during 13 of the 14 GLEs recorded in cycle 23 (as of August 2005). The GLE-associated CMEs represent the fastest known population of CMEs. All the GLEs were also associated with metric type II bursts. Comparison between GLE and metric type II onsets suggests that coronal shocks are formed before GLEs are released at the Sun. These results are consistent with particle acceleration by CME-driven shocks.

A preprint of this paper can be downloaded from NASA/ADS.


Introduction to violent Sun-Earth connection events of October-November 2003

N. Gopalswamy, L. Barbieri, E. W. Cliver, G. Lu, S. P. Plunkett, and R. M. Skoug
JGR, Vol. 110, No. A9, A09S00, doi:10.1029/2005JA011268, 2005

Abstract

The solar-terrestrial events of late October and early November 2003, popularly 7 referred to as the Halloween storms, represent the best observed cases of extreme space 8 weather activity observed to date and have generated research covering multiple aspects of 9 solar eruptions and their space weather effects. In the following article, which serves as 10 an abstract for this collective research, we present highlights taken from 61 of the 74 11 papers from the Journal of Geophysical Research, Geophysical Research Letters, and 12 Space Weather which are linked under this special issue. (An overview of the 13 13 associated papers published in Geophysics Research Letters is given in the work of 14 Gopalswamy et al. (2005a)).

See also NASA/ADS.


Coronal Mass Ejections and Other Extreme Characteristics of the 2003 October-November Solar Eruptions

N. Gopalswamy, S. Yashiro, Y. Liu, G. Michalek, A. Vourlidas, M. L. Kaiser, and R. A. Howard
JGR, Vol. 110, No. A9, A09S15, doi:10.1029/2004JA010958, 2005

Abstract

The violent solar eruptions occurring from 18 October to 8 November 2003 can be considered as extreme events in terms of both their source properties at the Sun and their heliospheric consequences. The eruptions were accompanied by intense solar flares and coronal mass ejections (CMEs) of very high energy. The plasma, particle and electromagnetic consequences of these events were detected by various instruments located throughout the heliosphere. Disturbances associated with two of the eruptions arrived at Earth in less than a day, providing bench mark data for space weather purposes. Historically, there were only 13 documented eruptions, with < 1-day shock travel time to Earth [Cliver et al., 1990a,b; Cliver and Svalgaard, 2004], including the 1859 September 1 event corresponding to the first flare ever reported [Carrington, 1860; Hodgson, 1860]. The purpose of this study is to obtain the statistical properties of the 2003 October-November CMEs and compare them with those of the general population of CMEs observed during solar cycle 23. This study also places these events in perspective of other large events. We analyze the flares, CMEs, shocks, and SEPs, all of which were linked to the magnetic free energy available in the underlying solar active regions. We also study the travel time of the associated shocks to Earth and the intensity of the consequent geomagnetic storms. We pay particular attention to the three fastest shocks of the study period that were followed by interplanetary CMEs and marked the commencement of intense geomagnetic storms. We finally examine the place of the 2003 October-November active regions among other SEP-producing regions of solar cycle 23.

A preprint of this paper can be downloaded as a pdf file ( manuscript)


Solar source of the largest geomagnetic storm of cycle 23

N. Gopalswamy, S. Yashiro, G. Michalek, H. Xie, R. P. Lepping, and R. A. Howard
GRL, Vol. 32, No. 12, L12S09, doi:10.1029/2004GL021639, 2005

Abstract

The largest geomagnetic storm of solar cycle 23 occurred on 2003 November 20 with a Dst index of -472 nT, due to a coronal mass ejection (CME) from active region 0501. The CME near the Sun had a sky-plane speed of ~1660 km/s, but the associated magnetic cloud (MC) arrived with a speed of only 730 km/s. The MC at 1 AU (ACE Observations) had a high magnetic field (~56 nT) and high inclination to the ecliptic plane. The southward component of the MC s magnetic field was made up almost entirely of its axial field because of its east-south-west (ESW) chirality. We suggest that the southward pointing strong axial field of the MC reconnected with Earth s front-side magnetic field, resulting in the largest storm of the solar cycle 23.

A preprint of this paper can be downloaded as a pdf file.


CME Interaction and the Intensity of Solar Energetic Particle Events

N. Gopalswamy, S. Yashiro, S. Krucker, and R. A. Howard
in proceeding of IAU Symposium No. 226, pp. 367-373, 2005

Abstract

Large Solar Energetic Particles (SEPs) are closely associated with coronal mass ejections (CMEs). The significant correlation observed between SEP intensity and CME speed has been considered as the evidence for such a close connection. The recent finding that SEP events with preceding wide CMEs are likely to have higher intensities compared to those without was attributed to the interaction of the CME-driven shocks with the preceding CMEs or with their aftermath. It is also possible that the intensity of SEPs may also be affected by the properties of the solar source region. In this study, we found that the active region area has no relation with the SEP intensity and CME speed, thus supporting the importance of CME interaction. However, there is a significant correlation between flare size and the active region area, which probably reflects the spatial scale of the flare phenomenon as compared to that of the CME-driven shock.

A preprint of this paper can be downloaded from NASA/ADS.


2004

Kinematics of coronal mass ejections between 2 and 30 solar radii:
What can be learned about forces governing the eruption?

B. Vrsnak, D. Rudjak, D. Sudar, and N. Gopalswamy
Astronomy and Astrophysics, 423, 717-728 (2004)

Abstract

Kinematics of more than 5000 coronal mass ejections (CMEs) measured in the distance range 2 30 solar radii is investigated. A distinct anticorrelation between the acceleration, a, and the velocity, v, is found. In the linear form, it can be represented as a = k1(v-v0), where v0 =400 km/s, i.e., most of the CMEs faster than 400 km/s decelerate, whereas slower ones generally accelerate. After grouping CMEs into the width and mean-distance bins, it was found that the slope k1 depends on these two parameters: k1 is smaller for CMEs of larger width and mean-distance. Furthermore, the obtained CME subsets show distinct quadratic-form correlations, of the form a = k2(v-v0)|v-v0|.The value of k2 decreases with increasing distance and width, whereas v0 increases with the distance and is systematically larger than the slow solar wind speed by 100-200 km/s. The acceleration-velocity relationship is interpreted as a consequence of the aerodynamic drag. The excess of v0 over the solar wind speed is explained assuming that in a certain fraction of events the propelling force is still acting in the considered distance range. In most events the inferred propelling force acceleration at 10 solar radii ranges between aL =0 and 10 m/s^2, being on average smaller at larger distances. However, there are also events that show aL >50 m/s^2, as well as events indicating aL <0. Implications for the interplanetary motion of CMEs are discussed, emphasizing the prediction of the 1 a.u. arrival time.

See also NASA/ADS.


Association of Coronal Mass Ejections and Type II Radio Bursts with Impulsive Solar Energetic Particle Events

S. Yashiro, N. Gopalswamy, E. W. Cliver, D. V. Reames, M. L. Kaiser, and R. A. Howard
in The Solar-B Mission and the forefront of Solar Physics, edited by T. Sakurai and T. Sekii, ASP Conference Series, Vol 325, pp. 401-408, 2004

Abstract

We report the association of impulsive solar energetic particle (SEP) events with coronal mass ejections (CMEs) and metric type II radio bursts. We identified 38 impulsive SEP events using the Wind/EPACT instrument and their CME association was investigated using white light data from SOHO/LASCO. We found that (1) at least ~ 28-39% of impulsive SEP events were associated with CMEs, (2) only 8-13% were associated with metric type II radio bursts. The statistical properties of the associated CMEs were investigated and compared with those of general CMEs and CMEs associated with large gradual SEP events. The CMEs associated with impulsive SEP events were significantly slower (median speed of 613 km/s) and narrower (49 deg) than those of CMEs associated with large gradual SEP events (1336 km/s, 360 deg), but faster than the general CMEs (408 km/s).

A preprint of this paper can be downloaded from NASA/ADS.


Intensity Variation of Large Solar Energetic Particle Events Associated with Coronal Mass Ejections

N. Gopalswamy, S. Yashiro, S. Krucker, G. Stenborg, and R. A. Howard
Journal of Geophysical Research, 109, A12105, doi:10.1029/2004JA010602

Abstract

We studied the coronal mass ejections (CMEs) and flares associated with large solar energetic particle (SEP) events of solar cycle 23 (1996-2002) in order to determine what property of the solar eruptions might order the SEP intensity. The SEP events were divided into three groups: (i) events in which the primary CME was preceded by one or more wide CMEs from the same solar source, (ii) events with no such preceding CMEs, and (iii) events in which the primary CME might have interacted with a streamer, or with a nearby halo CME. The SEP intensities are distinct for groups (i) and (ii) although the CME properties were nearly identical. Group (iii) was similar to group (i). The primary findings of this study are (1) Higher SEP intensity results whenever a CME is preceded by another wide CME from the same source region. (2) The average flare size was also larger for high intensity SEP events. (3) The intensity of SEP events with preceding CMEs showed a tighter correlation with CME speed. The extent of scatter in the CME speed vs. SEP intensity plots was reduced when various subgroups were considered separately. (4) The intensity of energetic electrons were better correlated with flare size than with CME speed. (5) The SEP intensity showed poor correlation with the flare size, except for group (iii) events. Since only a third of the events did not have preceding CMEs, we conclude that the majority of SEP producing CMEs propagate through the near-Sun interplanetary medium severely disturbed and distorted by the preceding CMEs. Furthermore, the preceding CMEs are faster and wider on the average, so they may provide seed particles for CME-driven shocks that follow. Therefore, we conclude that the differing intensities of SEP events in the two groups may not have resulted due to the inherent properties of the CMEs. The presence of preceding CMEs seems to be the discriminating characteristic of the high and low intensity SEP events.

A preprint of this paper can be downloaded as a pdf file (manuscript.


A global picture of CMEs in the inner heliosphere

N. Gopalswamy
in "The Sun and the Heliosphere as an Integrated system", ASSL series, ed. G. Poletto and S. Suess, KLUWER/Boston, Chapter 8, p. 201, 2004

Abstract

This is an overview of Coronal mass ejections (CMEs) in the heliosphere with an observational bias towards remote sensing by coronagraphs. Particular emphasis will be placed on the results from the Solar and Heliospheric Observatory (SOHO) mission which has produced high quality CME data uniform and continuos over the longest stretch ever. After summarizing the morphological, physical, and statistical properties of CMEs, a discussion on the phenomena associated with them is presented. These are the various manifestations of CMEs observed at different wavelengths and the accompanying phenomena such as shocks and solar energetic particles that provide information to build a complete picture of CMEs. Implications of CMEs for the evolution of the global solar magnetic field are presented. CMEs in the heliosphere are then discussed including out-of-the-ecliptic observations from Ulysses and the possibility of a 22-year cycle of cosmic ray modulation by CMEs. After outlining some of the outstanding questions, a summary of the chapter is provided.

See also NASA/ADS.


Interplanetary Radio Bursts

N. Gopalswamy
in Solar and Space Weather Radiophysics, Current Status and Feature Development, ASSL series, ed. D. Gary and C. Keller, Kluwer, p. 305, 2004

Abstract

Nonthermal radio bursts in the interplanetary medium indicate the far-reaching effect of solar eruptions that inject energetic particles, plasmas and shock waves into the inner heliosphere. More than half a century of ground-based observations and subsequent space-based observations exist on this phenomena. In this paper, I summarize the understanding we have gained on the type III and type II radio bursts, which are indicative of electron beams and shocks, respectively. Observations in the new radio window (1-14 MHz) from Wind/WAVES have not only confirmed previous results, but also led to a number of new discoveries. Availability of simultaneous white light (SOHO) and radio (Wind) observations from the same spatial domain in the near-Sun IP medium is largely responsible for these discoveries on the IP propagation of CMEs, so this paper discusses radio bursts in the context of white light observations. After exploring the origin of normal, complex and storm type III bursts, I discuss the type II bursts and their relation to coronal mass ejections. Finally I discuss some of the recent developments on IP radio emission.

A preprint of this paper can be downloaded as a pdf file.


Influence of CME Interaction on Propagation of Interplanetary Shocks

P.K. Manoharan, N. Gopalswamy, S. Yashiro, A. Lara, G. Michalek, and R. A. Howard (2004)
Journal of Geophysical Research, 109, A06109, doi:10.1029/2003JA010300

Abstract

We studied 91 interplanetary (IP) shocks associated with coronal mass ejections (CMEs) originating within about ± 30 degree in longitude and latitude from the center of the Sun during 1997 - 2002. These CMEs cover a wide range of initial speeds of about 120 to 2400 km/s and they also include a special population of 25 interacting CMEs. This study provides the characteristics of propagation effects of more number of high-speed CMEs (VCME > 1500 km/s) than the data used in earlier studies. It enables to extend the shock-arrival prediction model to high-speed CMEs. The results on comparison of IP shock speed and transit time at 1 AU suggest that the shock transit time is not controlled by its final speed but is primarily determined by the initial speed of the CME and effects encountered by it during the propagation. It is found that the CME interaction tends to slow the shock and associated CME. The deviations of shock arrival times from the empirical model are considerably large for slow (VCME < 300 km/s) and fast (VCME > 800 km/s) CMEs. Results show that the slow and fast CMEs experience stronger effective acceleration.

See also NASA/ADS.


A Catalog of White Light Coronal Mass Ejections Observed by the SOHO Spacecraft

S. Yashiro, N. Gopalswamy, G. Michalek, O. C. St.Cyr,
S. P. Plunkett, N. B. Rich, and R. A. Howard (2004)
Journal of Geophysical Research, 109, A07105, doi:10.1029/2003JA010282

Abstract

The Solar and Heliospheric Observatory (SOHO) mission's white light coronagraphs have observed nearly 7000 coronal mass ejections (CMEs) between 1996 and 2002. We have documented the measured properties of all these CMEs in an online catalog. We describe this catalog and present a summary of the statistical properties of the CMEs. The primary measurements made on each CME are the apparent central position angle, the angular width in the sky plane, and the height (heliocentric distance) as a function of time. The height-time measurements are then fitted to first and second order polynomials to derive the average apparent speed and acceleration of the CMEs. The statistical properties of CMEs are: (1) the average width of normal CMEs (20 < width < 120) increased from 47 deg (1996; solar minimum) to 61 deg (1999; early phase of solar maximum) and then decreased to 53 deg (2002; late phase of solar maximum), (2) CMEs were detected around the equatorial region during solar minimum, while during solar maximum CMEs appear at all latitudes, (3) the average apparent speed of CMEs increases from 300 km/s (solar minimum) to 500 km/s (solar maximum), (4) the average apparent speed of halo CMEs (957 km/s) is twice of that of normal CMEs (428 km/s), and (5) most of the slow CMEs (V < 250 km/s) show acceleration while most of the fast CMEs (V>900 km/s) show deceleration. Solar cycle variation and statistical properties of CMEs are revealed with greater clarity in this study as compared to previous studies. Implications of our findings for CME models are discussed.

See also NASA/ADS.


Recent Advances in the Long-Wavelength Radio Physics of the Sun

N. Gopalswamy
Planetary and Space Science, Vol 52 (15), p. 1399, 2004

Abstract

Solar radio bursts at long wavelengths provide information on the solar disturbances such as coronal mass ejections and shocks at the moment of their departure from the Sun. The radio bursts also provide information on the physical properties (density, temperature and magnetic field) of the medium that supports the propagation of the disturbances with a valuable cross-check from direct imaging of the quiet outer corona. The primary objective of this paper is to review some of the past results and highlight recent recent results obtained from long-wavelength observations. In particular, the discussion will focus on radio phenomena occurring in the outer corona and beyond in relation to those observed in white light. Radio emission from nonthermal electrons confined to closed and open magnetic structures and in large-scale shock fronts will be discussed with particular emphasis on its relevance to solar eruptions. Solar cycle variation of the occurrence rate of shock-related radio bursts will be discussed in comparison with those of interplanetary shocks and solar proton events. Finally, case studies describing the newly-discovered radio signatures of interacting CMEs will be presented.

A preprint of this paper can be downloaded as a pdf file.


Characteristics of coronal mass ejections in the near Sun
interplanetary space

A. Lara, J. A. Gonzalez-Esparza, and N. Gopalswamy
Geofisica Internacional, Vol. 43, Num. 1, pp. 75-82, 2004

Abstract

Based on the observations from the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO) spacecraft we studied coronal mass ejections (CME) parameters between 6 and 10 solar radii from the Sun. We have developed a new method to obtain the duration, brightness enhancement (above the background) and speed of the leading part of the CMEs. These properties are important in order to understand the dynamics of the CMEs near the Sun and their evolution in the interplanetary space using numerical models. We present the method of analysis and the results of the application of this method to a set of 9 halo CMEs observed during 1997. We found that i) the CME speeds obtained with this method are in good agreement with measurements of other observers, ii) the average time duration of the CME leading part is 8 hrs. and iii) the brightness maxima decrease with distance as a power law with a mean index of -3.6.


Forecast of solar ejecta arrival at 1 AU from radial speed

S. Dasso, N. Gopalswamy, and A. Lara
Geofisica Internacional, Vol. 43, Num. 1, pp. 47-52, 2004

Abstract

Solar ejecta produce changes in the interplanetary magnetic field of the terrestrial environment. When the magnetic polarity of the ejecta is suitable, it may trigger intense geomagnetic storms. Therefore, prediction of the arrival of solar ejecta in the geospace is of crucial importance for space weather applications. We implement a simple model, developed by Gopalswamy et al., (2000) to estimate the time of arrival for solar ejecta at 1AU. This model requires just one input parameter: the radial speed of the associated coronal mass ejection (CME) at the moment of its expulsion from the Sun. When the speed of the CME is measured from a location on the Sun-Earth line, only the plane of the sky speed can be obtained. Since the prediction model depends on the initial speed of the CMEs observed remotely, it is important to obtain this speed as accurately as possible. One of the major uncertainties in the measured initial speed is the extent of projection effects. We attempt to correct for projection effects using the solar surface location of the eruption and assuming a width to the CME. We found that the correction is in agreement with a model obtained from stereoscopic observations from the past.


VARIABILITY OF SOLAR ERUPTIONS DURING CYCLE 23

N. Gopalswamy, S. Nunes, S. Yashiro, And R. A. Howard
Adv. Space. Res., Vol. 34, Issue 2, p. 391-396, 2004

Abstract

We report on the solar cycle variation of the rate of coronal mass ejections (CMEs), their mean and median speeds, and the rate of type II radio bursts. We found that both CME rate and speed (mean and median) increased from solar minimum to maximum by factors of 10 and 2, respectively. The CME rate during solar maximum is nearly twice the rates quoted previously. Large spikes in the speed variation were due to active regions that were highly active. The poor correlation between metric and DH type II bursts is confirmed, and the difference is attributed to the different Alfven speeds in the respective source regions.

A preprint of this paper can be downloaded as a pdf file.


On Coronal Streamer Changes

N. Gopalswamy, M. Shimojo, W. Lu, S. Yashiro, K. Shibasaki, and R. A. Howard
Adv. Space Res., Vol. 33, Issue 5, p. 676-680, 2004

Abstract

Coronal streamer represents one of the pre-eruption configurations of coronal mass ejections (CMEs), because they overlie prominences and often possess all the substructures of CMEs. In this paper, we report on a study of streamer changes associated with prominence eruptions. The prominence eruptions and streamer changes were observed by the Nobeyama radioheliograph and Solar and Heliospheric Observatory (SOHO), respectively. Multiwavelength data showed that at least one of the streamer events involved heating and small-scale material ejection that subsequently stalled. After presenting illustrative examples, we compare the properties of the streamer-related events with those of general population of prominence events. We find that the properties of streamer-related prominence events are closer to those of prominence eruptions with transverse trajectories.

A preprint of this paper can be downloaded as a MS-Word file.


2003

A New Method for Estimating Widths, Velocities, and Source Location of Halo Coronal Mass Ejections

G. Michalek, N. Gopalswamy, and S. Yashiro
ApJ, Volume 584, Issue 1, pp.472-478, 2003

Abstract

It is well known that coronagraphic observations of halo coronal mass ejections (CMEs) are subject to projection effects. Viewing in the plane of the sky does not allow us to determine the crucial parameters that define the geoeffectiveness of CMEs, such as the space speed, width, or source location. Assuming that halo CMEs have constant velocities, are symmetric, and propagate with constant angular widths, at least in their early phase, we have developed a technique that allows us to obtain the required parameters. This technique requires measurements of sky-plane speeds and the moments of the first appearance of the halo CMEs above opposite limbs. We apply this technique to obtain the parameters of all the halo CMEs observed by the Solar and Heliospheric Observatory (SOHO) mission's Large Angle and Spectrometric Coronagraph experiment until the end of 2000. We also present a statistical summary of these derived parameters of the halo CMEs.

A preprint of this paper can be downloaded from a NASA/ADS.


Comment on "Coronal mass ejections, interplanetary ejecta and geomagnetic storms?" by H. V. Cane, I. G. Richardson, and O. C. St. Cyr

N. Gopalswamy, P. K. Manoharan, and S. Yashiro
GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 24, 2232, doi:10.1029/2003GL017562, 2003

Abstract

Cane et al. [2000] claimed that the majority of the interplanetary ejecta (IP) in their study arrive at 1 AU earlier than is predicted by the empirical model of Gopalswamy et al. [2000]. We show that this claim is not valid because the transit times they used are not for ejecta, but for a "mixed bag" containing sheaths ahead of the IP ejecta and some ejecta.

See also NASA/ADS.


CORONAL MASS EJECTIONS AND SOLAR POLARITY REVERSAL

N. Gopalswamy, A. Lara, S. Yashiro, And R. A. Howard
The Astrophysical Journal, 598, L63, 2003

Abstract

We report on a close relationship between the solar polarity reversal and the cessation of high-latitude coronal mass ejections (CMEs). This result holds good for individual poles of the Sun for cycles 21 and 23, for which CME data are available. The high-latitude CMEs provide a natural explanation for the disappearance of the polar crown filaments (PCFs) that rush the poles. The PCFs, which are closed field structures, need to be removed before the poles could acquire open field structure of the opposite polarity. Inclusion of CMEs along with the photospheric and subphotospheric processes completes the full set of phenomena to be explained by any solar dynamo theory.

See also NASA/ADS.


CORONAL MASS EJECTION INTERACTION AND PARTICLE ACCELERATION DURING THE 2001 APRIL 14-15 EVENTS

N. Gopalswamy, S. Yashiro, M. L. Kaiser, and R. A. Howard
Adv. Space. Res., Vol 32, Issue 12, p. 2613-2618, 2003

Abstract

Two successive solar energetic particle (SEP) events associated with fast and wide coronal mass ejections (CMEs) on 2001 April 14 and 15 are compared. The weak SEP event of April 14 associated with an 830 km/s CME and an M1.0 flare was the largest impulsive event of cycle 23. The April 15 event, the largest ground level event of cycle 23, was three orders of magnitude more intense than the April 14th event and was associated with a faster CME (1200 km/s) and an X14.4 flare. We compiled and compared all the activities (flares, CMEs, interplanetary conditions and radio bursts) associated with the two SEP events to understand the intensity difference between them. Different coronal and interplanetary environments of the two events (presence of preceding CME and seed particles ahead of the April 15 event) may explain the intensity difference.

A preprint of this paper can be downloaded as a pdf file.


Coronal Mass Ejection Activity During Solar Cycle 23

N. Gopalswamy, A. Lara, S. Yashiro, S. Nunes, and R. A. Howard
Proceeding of Solar Variability as an input to the Earth's Environment, ESA-SP, p. 403-414, 2003

Abstract

We studied the solar cycle variation of various properties of coronal mass ejections (CMEs), such as daily CME rate, mean and median speeds, and the latitude of solar sources for cycle 23 (1996-2002). We find that (1) there is an order of magnitude increase in CME rate from the solar minimum (0.5/day) to maximum (6/day), (2) the maximum rate is significantly higher than previous estimates, (3) the mean and median speeds of CMEs also increase from minimum to maximum by a factor of 2, (4) the number of metric type II bursts (summed over CR) tracks CME rate, but the CME speed seems to be only of secondary importance, (5) for type II bursts originating farther from the Sun the CME speed is important, (6) the latitude distribution of CMEs separate the prominence-associated (high-latitude) and active-region associated CMEs, and (7) the rate of high-latitude CMEs shows north-south asymmetry and the cessation eruptions in the north and south roughly mark the polarity reversals. We compared the rates of the fast-and-wide CMEs, major solar flares, interplanetary (IP) shocks, long-wavelength type II bursts and large SEP events. This comparison revealed that the number of major flares is generally too large compared to all the other numbers. In other words, fast-and-wide CMEs, long-wavelength type II bursts, large SEP events, and IP shocks have a close physical relationship.

A preprint of this paper can be downloaded as a pdf file.


Coronal and Interplanetary Environment of Large Solar Energetic Particle Events

Nat Gopalswamy, Seiji Yashiro, Guillermo Stenborg, and Russell Howard
Proceeding of 28th International Cosmic Ray Conference, p. 3549, 2003

Abstra#t

We studied the properties of coronal mass ejections (CMEs)associated with large solar energetic particle (SEP)events during 1997-2002 and compared them with those of preceding CMEs from the same source region.The primary ndings of this study are (1)High-intensity (> 50 protons cm-2s-1sr-1 )events are more likely to be preceded by other wide CMEs.(2)The preceding CMEs are faster and wider than average CMEs.(3)The primary CMEs often propagate through the near-Sun interplanetary medium severely disturbed and distorted by the preceding CMEs.

A preprint of this paper can be downloaded from NASA/ADS.


Solar and geospace connections of energetic particle events

N. Gopalswamy
GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 12, 8013, doi:10.1029/2003GL017277, 2003

Abstract

A Coordinated Data Analysis Workshop (CDAW) was conducted recently to study the solar and geospace connections of large solar energetic particle (SEP) events of solar cycle 23 (up to the end of 2001). This paper summarizes the properties these events, the scientific issues discussed, and some of the results obtained during the workshop.

A preprint of this paper can be downloaded from GRL site as a pdf file.


Large solar energetic particle events of cycle 23: A global view

N. Gopalswamy, S. Yashiro, A. Lara, M. L. Kaiser, B. J. Thompson, P. T. Gallagher, and R. A. Howard
GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 12, 8015, doi:10.1029/2002GL016435, 2003

Abstract

We report on a study of all the large solar energetic particle (SEP) events that occurred during the minimum to maximum interval of solar cycle 23. The main results are: 1. The occurrence rate of the SEP events, long-wavelength type II bursts and the fast and wide frontside western hemispheric CMEs is quite similar, consistent with the scenario that CME-driven shocks accelerate both protons and electrons; major flares have a much higher rate. 2. The SEP intensity is better correlated with the CME speed than with the X-ray flare class. 3. CMEs associated with high-intensity SEPs are about 4 times more likely to be preceded by wide CMEs from the same solar source region, suggesting the importance of the preconditioning of the eruption region. We use a specific event to demonstrate that preceding eruption from a nearby source can significantly affect the properties of SEPs and type II radio bursts.

A preprint of this paper can be downloaded from GRL site as a pdf file.


A statistical study of CMEs associated with metric type II bursts

A. Lara, N. Gopalswamy, S. Nunes, G. Munoz, and S. Yashiro
GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 12, 8016, doi:10.1029/2002GL016481, 2003

Abstract

We present a statistical study of the characteristics of CMEs which show temporal association with type II bursts in the metric domain but not in the decameter/hectometric (DH) domain. This study is based on a set of 80 metric (m) type II bursts associated with surface events in the solar western hemisphere. It was found that in general, the distribution of the widths and speeds of the CMEs associated with metric (but not DH) type II bursts are shifted towards higher values compared to those of all CMEs observed by LASCO in the 1996-2001 period. We also found that these distributions have lower values than the same distributions of the CMEs associated with DH type II bursts. In terms of energy, this means that the CMEs associated only with metric type II bursts are more energetic (wider and faster) than regular CMEs but less energetic than the CMEs associated with DH type II bursts.

A preprint of this paper can be downloaded from GRL site as a pdf file.


Long-duration hectometric type III radio bursts and their association with solar energetic particle (SEP) events

R. J. MacDowall, A. Lara, P. K. Manoharan, N. V. Nitta, A. M. Rosas, J. L. Bougeret
GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 12, 8018, doi:10.1029/2002GL016624, 2003

Abstract

It has recently been suggested by Cane et al. [2002] that a class of type III solar radio bursts, called type III-l, is reliably associated with intense solar energetic particle (SEP) events. They proposed that the causative electrons for these bursts are accelerated in regions of reconnecting magnetic field in the wakes of coronal mass ejections (CMEs). In this paper, we examine the durations, intensities, and other characteristics of such radio bursts in the hectometric frequency range and compare them to several groups of control events. We conclude that simple criteria, based on hectometric data alone, can identify the majority (~80%) of type III-l radio bursts, which are associated with >20 MeV SEP proton events, while excluding almost 100% of the control events. Detailed study of these type III-l bursts may play a significant role in a better understanding of the acceleration of SEPs and of the magnetic field evolution in the vicinity of CMEs.

A preprint of this paper can be downloaded from GRL site as a pdf file.


Properties of Narrow Coronal Mass Ejections Observed with LASCO

S. Yashiro, N. Gopalswamy, G. Michalek, and R. A. Howard

Advances in Space Research, 32(12), pp. 2631-2635, 2003

Abstract

We report the statistical properties of narrow coronal mass ejections (CMEs, angular width are < 20 deg) with particular emphasis on comparison with normal CMEs. We investigated 806 narrow CMEs from an online LASCO/CME catalog and found that (1) the fraction of narrow CMEs increases from 12% to 22% towards solar maximum, (2) during the solar maximum, the narrow CMEs are generally faster than normal ones, (3) the maximum speed of narrow CMEs (1141 km/s) is much smaller than that of the normal CMEs (2604 km/s). These results imply that narrow CMEs do not form a subset of normal CMEs and have a different acceleration mechanism from normal CMEs.

A preprint of this paper can be downloaded as a pdf file.


Effect of CME Interactions on the Production of Solar Energetic Particles

N. Gopalswamy, S. Yashiro, G. Michalek, M. L. Kaiser, R. A. Howard, R. Leske, T. von Rosenvinge, and D. V. Reames

Solar Wind X, ed. M. Velli, AIP Conf., Vol. 679, pp. 608-611, 2003

Abstract

We analyzed a set of 52 fast and wide, frontside western hemispheric (FWFW) CMEs in conjunction with solar energetic particle (SEP) and radio burst data and found that 42 of these CMEs were associated with SEPs. All but two of the 42 SEP-associated FWFW CMEs (95%) were interacting with preceding CMEs or dense streamers. Most of the remaining 10 SEP-poor FWFW CMEs had either insignificant or no interaction with preceding CMEs or streamers, and were ejected into a tenuous corona. There is also a close association between type II radio bursts in the near-Sun interplanetary medium and SEP-associated FWFW CMEs suggesting that electron accelerators are also good proton accelerators.

A preprint of this paper can be downloaded as a pdf file.


Prominence Eruptions and Coronal Mass Ejection: A Statistical Study using Microwave Observations

N. Gopalswamy, M. Shimojo, W. Lu, S. Yashiro, K. Shibasaki, and R. A. Howard

The Astrophysical Journal, Vol 586, pp 562-578, 2003.

Abstract

We present the results of a statistical study of a large number of solar prominence events (PEs) observed by the Nobeyama Radioheliograph. We studied the association rate, relative timing and spatial correspondence between PEs and coronal mass ejections (CMEs). We classified the PEs as radial and transverse, depending on whether the prominence moved predominantly in the radial or horizontal direction. The radial events were faster and attained a larger height above the solar surface than the transverse events. Out of the 186 events studied, 152 (82%) were radial events, while only 34 (18%) were transverse events. Comparison with white-light CME data revealed that 134 (72%) PEs were clearly associated with CMEs. We compare our results with those of other studies involving PEs and white light CMEs in order to address the controversy in the rate of association between CMEs and prominence eruptions. We also studied the temporal and spatial relationship between prominence and CME events. The CMEs and PEs seem to start roughly at the same time. There was no solar cycle dependence of the temporal relationship. The spatial relationship was, however, solar cycle dependent. During the solar minimum, the central position angle of the CMEs had a tendency to be o set closer to the equator as compared to to that of the PE, while no such e ect was seen during solar maximum.

A preprint of this paper can be downloaded as a pdf file.


Coronal Mass Ejections: Initiation and Detection

N. Gopalswamy

Adv. Space Res., Vol. 31, Issue 4, p. 869-881, 2003

Abstract

Coronal mass ejections (CMEs) are large-scale magnetic structures expelled from the Sun due to MHD processes involving interaction between plasma and magnetic field in closed flux regions. I provide a summary of the observational signatures and current models on CME initiation. CMEs are traditionally observed using white light coronagraphs. I also provide a summary of various signatures of CMEs detected in other wavelengths, which have helped us obtain a complete picture of the CME phenomenon in the inner heliosphere.

A preprint of this paper can be downloaded as a pdf file.


2002

Interacting Coronal Mass Ejections and Solar Energetic Particles

N. Gopalswamy, S. Yashiro, G. Michalek, M. L. Kaiser, R. A. Howard, D. V. Reames, R. Leske, and T. von Rosenvinge

The Astrophysical Journal, Volume 572, Issue 1, pp. L103-L107, 2002.

Abstract

We studied the association between solar energetic particle (SEP) events and coronal mass ejections (CMEs) and found that CME interaction is an important aspect of SEP production. Each SEP event was associated with a primary CME that is faster and wider than average CMEs and originated from west of E45. For most of the SEP events, the primary CME overtakes one or more slower CMEs within a heliocentric distance of ~ 20 Rsun. In an inverse study, we found that for all the fast (speed > 900 km/s) and wide (width > 60 deg) western hemispheric frontside CMEs during the study period, the SEP-associated CMEs were ~ 4 times more likely to be preceded by CME interaction than the SEP-poor CMEs. i.e., CME interaction is a good discriminator between SEP-poor and SEP-associated CMEs. We infer that the efficiency of the CME-driven shocks is enhanced as they propagate through the preceding CMEs and that they accelerate SEPs from the material of the preceding CMEs rather than from the quiet solar wind. We also found a high degree of association between major SEP events and interplanetary type II radio bursts suggesting that proton accelerators are also good electron accelerators.


See also NASA/ADS


Relation between CMEs and ICMEs

N. Gopalswamy

in Solar-Terrestrial Magnetic Activity and Space Enviroment, COSPAR Colloquia Series, Vol. 14, edited by H. N. Wang and R. L. Xu, p. 157, 2002.

Abstract

Our current knowledge on coronal mass ejections (CMEs) comes from two spatial domains: the near-Sun (up to 30 solar radii) region remote-sensed by coronagraphs and the geospace and beyond where in situ observations are made by spacecraft. Comparing observations from these two domains has helped us understand the propagation and evolution of CMEs through the interplanetary (IP) medium and develop an empirical model to predict the 1-AU arrival of CMEs. In this paper, we review the available information on the relation between CMEs and their IP counterparts. In particular, we concentrate on issues related to the prediction of the arrival of ICMEs in the geospace. We discuss the solar sources of the three largest geomagnetic storms of year 2000 and compare the predicted and observed arrival times of the associated CMEs.

A preprint of this paper can be downloaded as a pdf file or a MS-Word file.


Statistical Properties of Radio-rich Coronal Mass Ejections

N. Gopalswamy, S. Yashiro, G. Michalek, M. L. Kaiser and R. A. Howard

in Solar-Terrestrial Magnetic Activity and Space Enviroment, COSPAR Colloquia Series, Vol. 14, edited by H. N. Wang and R. L. Xu, p. 173, 2002.

Abstract

Coronal mass ejections (CMEs) that produce type II radio bursts in the near-Sun interplanetary medium are termed radio-rich owing to their ability to drive MHD shocks. We summarize the statistical properties of these CMEs in order to see if they constitute a special population distinct from the general population. We found that these CMEs are faster and wider than the regular CMEs and show significant deceleration within the coronagraph field of view. Most of these CMEs were also found to be proton accelerators. We conclude that these type II bursts may be indicative of geoeffective CMEs and hence relevant to space weather.

A preprint of this paper can be downloaded as a pdf file or a MS-Word file.


Coronal Mass Ejections and Their Geospace Consequences

N. Gopalswamy

in the Proc. of the Silver Jubilee Symposium of the Udaipur Solar Observatory, in press, 2002.

Abstract

I summarize the observed properties of CMEs and discuss two of their major consequences relevant to the geospace: geomagnetic storms and large solar energetic particle (SEP) events. The magnetic field structure of a CME essentially decides whether it can result in a geomagnetic storm. On the other hand, its shock-driving capability decides the production of SEPs. I also briefly discuss how advance warning of the arrival of CME related disturbances can be obtained. Finally, I touch upon the interacting CMEs, a new development in the study of the origin and propagation of CMEs.

A preprint of this paper can be downloaded as a pdf file.


Interplanetary radio emission due to interaction between two coronal mass ejections

Nat Gopalswamy, Seiji Yashiro, Michael L. Kaiser, Russell A. Howard, and J.-L. Bougeret

Geophysical Research Letters, 29(8), 10.1029/2001GL013606, 2002

Abstract

We report on the detection of a new class of nonthermal radio emission due to the interaction between two coronal mass ejections (CMEs). The radio emission was detected by the Radio and Plasma Wave Experiment (WAVES) on board the Wind satellite, while the CMEs were observed by the white-light coronagraphs of the Solar and Heliospheric Observatory (SOHO) mission. There was no type II radio burst (metric or interplanetary) preceding the nonthermal emission. The radio emission occurred at a distance beyond 10 R from the Sun, where the two CMEs came in contact. Using Hα and EUV images, we found that the two CMEs were ejected roughly along the same path. We argue that the nonthermal electrons responsible for the new type of radio emission were accelerated due to reconnection between the two CMEs and/or due to the formation of a new shock at the time of the collision between the two CMEs.

A preprint of this paper can be downloaded as a pdf file.
See also GRL Web page


SOLAR ERUPTIONS AND LONG WAVELENGTH RADIO BURSTS: THE 1997 MAY 12 EVENT

N. Gopalswamy and M. L. Kaiser

Advances in Space Research, Volume 29, Issue 3, p. 307-312.

Abstract

We report on the cause of the 1997 May 12 type II bursts observed by ground based and space-based radio instruments. We estimate the fast mode speed in the corona as a function of heliocentric distance to identify the regions where fast mode shocks can be driven by CMEs. We find that both the coronal and the interplanetary type II bursts can be explained by shocks driven by the same CME at two different spatial domains. The fast mode speed in the corona has a peak at a heliocentric distance of ~ 3 Rsun which does not allow the coronal shock wave to propagate beyond this distance. When the CME continues to travel beyond the fast mode peak, another shock forms in the interplanetary medium where the fast mode speed falls sufficiently. From the radio observations we can infer that the plane of the sky speed of the CME is smaller than the space speed by at least a factor of 2, consistent with the location of the eruption at N21 W08. The inferred CME speed is also consistent with previous deprojected speed estimates.

A preprint of this paper can be downloaded as a pdf file.
See also NASA ADS Service


SPACE WEATHER STUDY USING COMBINED CORONAGRAPHIC AND IN SITU OBSERVATIONS

N. Gopalswamy

Space Weather Study Using Multipoint Techniques, Proceedings of the COSPAR Colloquium held in Pacific Green Bay, Wanli, Taipei, Taiwan, 27-29 September, 2000. Edited by Ling-Hsiao Lyu. Pergamon Press, 2002., p.39

Abstract

Coronal mass ejections (CMEs) play an important role in space weather studies because of their ability to cause severe geoeffects, such as magnetic storms. Shocks driven by CMEs may also accelerate solar energetic particles. Prediction of the arrival of these CMEs is therefore of crucial importance for space weather applications. After a brief review of the prediction models currently available, a description of an empirical model to predict the 1 AU arrival CMEs is provided. This model was developed using two-point measurements: (i) the initial speeds and onset times of Earth-directed CMEs obtained by white-light coronagraphs, and (ii) the corresponding interplanetary CME speeds and onset times at 1 AU obtained in situ. The measurements yield an empirical relationship between the interplanetary acceleration faced by the CMEs and their initial speeds, which forms the basis of the model. Use of archival data from spacecraft in quadrature is shown to refine the acceleration versus initial speed relationship, and hence the prediction model. A brief discussion on obtaining the 1-AU speed of CMEs from their initial speeds is provided. Possible improvements to the prediction model are also suggested.

A preprint of this paper can be downloaded as a pdf file.


2001

Characteristics of coronal mass ejections associated with long wavelength type II radio bursts

N. Gopalswamy, S. Yashiro, M. L. Kaiser, R. A. Howard, and J.-L. Bougeret

Journal of Geophysical Research, Vol. 106 , No. A12 , p. 29,219 (2001)

Abstract

We investigated the characteristics of coronal mass ejections (CMEs) associated with long wavelength type II radio bursts in the near-Sun interplanetary medium. Type II radio bursts in the decameter-hectometric (DH) wavelengths indicate powerful MHD shocks leaving the inner solar corona and entering the interplanetary medium. Almost all of these bursts are associated with wider- and faster-than-average CMEs. A large fraction of these radio-rich CMEs were found to decelerate in the coronagraph field of view, in contrast to the prevailing view that most CMEs display either constant acceleration or constan t speed. We found a similar deceleration for the fast CMEs (speed > 900 km/s) in general. We suggest that the coronal dra g could be responsible for the deceleration, based on the result that the deceleration has a quadratic dependence on the CME speed. About 60% of the fast CMEs were not associated with DH type II bursts, suggesting that some additional condition needs to be sat isfied to be radio-rich. The average width (66°) of the radio-poor, fast CMEs is much smaller than that (102°) of the radio-rich CMEs, suggesting that the CME width pays an important role. The special characteristics of the radio-rich CMEs suggest th at the detection of DH radio bursts may provide a useful tool in identifying the population of geoeffective CMEs.

A preprint of this paper can be downloaded as a pdf file.
See also NASA ADS Service


Predicting the 1-AU Arrival Times of Coronal Mass Ejections

N. Gopalswamy, A. Lara, S. Yashiro, M. L. Kaiser, and R. A. Howard

Journal of Geophysical Research, Vol. 106 , No. A12 , p. 29,207 (2001)

Abstract

We describe an empirical model to predict the 1-AU arrival of coronal mass ejections (CMEs). This model is based on an effective interplanetary (IP) acceleration described in Gopalswamy et al. [2000b] that the CMEs are subject to, as they propagate from the Sun to 1 AU. We have improved this model (i) by minimizing the projection effects (using data from spacecraft in quadrature) in determining the initial speed of CMEs, and (ii) by allowing for the cessation of the interplanetary acceleration before 1 AU. The resulting effective IP acceleration was higher in magnitude than what was obtained from CME measurements from spacecraft along the Sun-Earth line. We evaluated the predictive capability of the CME arrival model using recent two-point measurements from the Solar and Heliospheric Observatory (SOHO), Wind and ACE spacecraft. We found that an acceleration cessation distance of 0.76 AU is in reasonable agreement with the observations. The new prediction model reduces the average prediction error from 15.4 to 10.7 hrs. The model is in good agreement with the observations for high speed CMEs. For slow CMEs, the model as well as observations show a flat arrival time of ~4.3 days. Use of quadrature observations minimized the projection effects naturally without the need to assume the width of the CMEs. However, there is no simple way of estimating the projection effects based on the surface location of the Earth-directed CMEs observed by a spacecraft (such as SOHO) located along the Sun-Earth line because it is impossible to measure the width of these CMEs. The standard assumption that the CME is a rigid cone may not be correct. In fact, the predicted arrival times have a better agreement with the observed arrival times when no projection correction is applied to the SOHO CME measurements. The results presented in this work suggest that CMEs expand and accelerate near the Sun (inside 0.7 AU) more than our model supposes; these aspects will have to be included in future models.

A preprint of this paper can be downloaded as a pdf file.
See also NASA ADS Service


Introduction to special section:
Global picture of solar eruptive events

N. Gopalswamy

Journal of Geophysical Research, Vol. 106 , No. A11 , p. 25,135 (2001)

Abstract

This introduction highlights some of the scientific results reported in this special section on solar eruptive events and provides a brief description of issues related to the new results. Most of these papers grew out of the coordinated data analysis workshop held at the Goddard Space Flight Center during April 27-30, 1999, and the subsequent International Conference on Solar Eruptive Events held at the Catholic University of America, Washington, D. C. during March 6-9, 2000.

A preprint of this paper can be downloaded as a pdf file.
See also NASA ADS Service


Near-Sun and near-Earth manifestations of solar eruptions

N. Gopalswamy, A. Lara, M. L. Kaiser, and J.-L. Bougeret

Journal of Geophysical Research, Vol. 106 , No. A11 , p. 25,261 (2001)

Abstract

We compare the near-Sun and near-Earth manifestations of solar eruptions that occurred during November 1994 to June 1998. We compared white-light coronal mass ejections, metric type II radio bursts, and extreme ultraviolet wave transients (near the Sun) with interplanetary (IP) signatures such as decameter-hectometric type II bursts, kilometric type II bursts, IP ejecta, and IP shocks. We did a two-way correlation study to (1) look for counterparts of metric type II bursts that occurred close to the central meridian and (2) look for solar counterparts of IP shocks and IP ejecta. We used data from Wind and Solar and Heliospheric Observatory missions along with metric radio burst data from ground-based solar observatories. Analysis shows that (1) most (93%) of the metric type II bursts did not have IP signatures, (2) most (80%) of the IP events (IP ejecta and shocks) did not have metric counterparts, and (3) a significant fraction (26%) of IP shocks were detected in situ without drivers. In all these cases the drivers (the coronal mass ejections) were ejected transverse to the Sun-Earth line, suggesting that the shocks have a much larger extent than the drivers. Shocks originating from both limbs of the Sun arrived at Earth, contradicting earlier claims that shocks from the west limb do not reach Earth. These shocks also had go/d type II radio burst association. We provide an explanation for the observed relation between metric, decameter-hectometric, and kilometric type II bursts based on the fast mode magnetosonic speed profile in the solar atmosphere.

A preprint of this paper can be downloaded as a pdf file.
See also NASA ADS Service


X-ray Ejecta, White Light CMEs and a Coronal Shock Wave

N. Gopalswamy, O. C. St. Cyr, M. L. Kaiser, and S. Yashiro

Solar Physics, v. 203, Issue 1, p. 149-163 (2001)

Abstract

We report on a coronal shock wave inferred from the metric type II burst of 1996 January 13. To identify the shock driver, we examined mass motions in the form of X-ray ejecta and white light coronal mass ejections (CMEs). None of the ejections could be considered fast (> 400 km/s) events. In white light, two CMEs occurred in quick succession, with the first one associated with an X-ray ejecta near the solar surface. The second CME started at an unusually large height in the corona and carried a dark void in it. The first CME decelerated and stalled while the second one accelerated, both in the coronagraph field of view. We identify the X-ray ejecta to be the driver of the coronal shock inferred from metric type II burst. The shock speed reported in the Solar Geophysical Data (1000-2000 km/s) seems to be extremely large compared to the speeds inferred from X-ray and white light observations. We suggest that the MHD fast-mode speed in the inner corona is low enough that the X-ray ejecta is supermagnetosonic and hence can drive a shock to produce the type II burst.

A preprint of this paper can be downloaded as a pdf file.
See also NASA ADS Service


Radio Signatures of Coronal Mass Ejection Interaction: Coronal Mass Ejection Cannibalism?

Gopalswamy, N.; Yashiro, S.; Kaiser, M. L.; Howard, R. A.; Bougeret, J.-L.

The Astrophysical Journal, Volume 548, Issue 1, pp. L91-L94.

Abstract

We report the first detection at long radio wavelengths of interaction between coronal mass ejections (CMEs) in the interplanetary medium. The radio signature is in the form of intense continuum-like radio emission following an interplanetary type II burst. At the time of the radio enhancement, coronagraphic images show a fast CME overtaking a slow CME. We interpret the radio enhancement as a consequence of shock strengthening when the shock ahead of the fast CME plows through the core of the preceding slow CME. The duration of the radio enhancement is consistent with the transit time of the CME-driven shock through the core of the slow CME. As a consequence of the interaction, the core of the slow CME changed its trajectory significantly. Based on the emission characteristics of the radio enhancement, we estimate the density of the core of the slow CME to be ~4×104 cm-3. The CME interaction has important implications for space weather prediction based on halo CMEs: some of the false alarms could be accounted for by CME interactions. The observed CME interact)on could also explain some of the complex ejecta at 1 AU, which have unusual composition.

Full article of this paper can be downloaded as a pdf file form the web site of Astrophysical Journal.
See also NASA ADS Service


Comments or questions, contact the CDAW Webmaster.