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. Makela: Refereed Journals, Proceedings, Meeting Abstracts
H. Joeng: Refereed Journals, Proceedings, Meeting Abstracts

Refereed Papers Making Use of CME Catalog Data

2009
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. Makela, 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. Makela
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 Proceed- ings, 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. Makela, 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.

Large Geomagnetic Storms Associated with Limb Halo Coronal Mass Ejections

Nat Gopalswamy, Seiji Yashiro, Hong Xie, Sachiko Akiyama, and Pertti Makela
accepted for publication in Advances in Geosciences 2009

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 from the arXiv.

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.

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

2008
Large Geomagnetic Storms: Introduction to Special Section

N. Gopalswamy
J. Geophys. Res. in press (2008)

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.

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

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 N112 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.

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

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.

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

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.

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

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., 2008 in press.

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.

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.

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

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.

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

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, in press, 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.

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

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.

A preprint of this paper can be downloaded as a pdf file (an online supplement table).

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.

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

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.

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

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.

A preprint of this paper can be downloaded as a pdf file (an online supplement table).

PROPERTIES OF INTERPLANETARY CORONAL MASS EJECTIONS

Nat Gopalswamy
Space Science Reviews, 2007. 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.

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

2006
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.

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

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.

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

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.

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

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.

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

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.

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

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.

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

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.

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

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.

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

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-α, 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 as a pdf file.

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.

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

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.

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

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 as a pdf file.

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.

A preprint of this paper can be downloaded as a 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


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

Long Lived Geomagnetic Storms and Coronal Mass Ejections

H. Xie, N. Gopalswamy, P.K. Manoharan, A. Lara, S. Yashiro, and S. Lepri
accepted by JGR

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.

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

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.

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

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.

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

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 as a pdf file.

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)).

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

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, table)

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 as a pdf file.

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.

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

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 as a pdf file.

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, table).

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.

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

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.

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

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.

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

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

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.

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

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 or a MS-Word 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 as a pdf file.

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.

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

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.

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

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 as a pdf file.

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.

A New Method for Estimating Widths, Velocities, and Source Location of Halo CMEs

G. Michalek, N. Gopalswamy, and S. Yashiro

The Astrophysical Journal, Vol. 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 e ects. Viewing in the plane of the sky does not allow us to determine the crucial parameters de ning geoe ectivness 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 which allows to obtain the required parameters. This technique requires measurements of skyplane speeds and the moments of the rst 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 (LASCO) 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 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 or a MS-Word 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.

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

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.

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.

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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.

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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.
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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.

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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.
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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.
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