SOHO LASCO CME CATALOG: A Brief Description
This catalog contains all CMEs manually identified since 1996 from the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO) mission. LASCO has three telescopes C1, C2, and C3. However, only C2 and C3 data are used for uniformity because C1 was disabled in June 1998. At the outset, we would like to point out that the list is necessarily incomplete because of the nature of identification. In the absence of a perfect automatic CME detector program, the manual identification is still the best way to identify CMEs. This data base will serve as a reference to validate automatic identification programs being developed.
The top-level of the catalog is a year-month matrix, each element giving the monthly lists of CMEs.
The monthly list contains most of the information assembled from measurements and compilation from online data bases. Entries in this list have links to additional information on CMEs. At the top of the monthly lists, simple explanation is provided for getting information from additional layers. Link to the list of data gaps during the month is also provided. Data gaps of duration 3 h or more are listed. The data-gap list must be consulted before deciding the existence or nonexistence of CMEs. If there is a data gap, it is difficult to say there was a CME or not during the data gap.
Each row in the monthly list corresponds to one CME. The first three columns of the monthly list serve as an ID for each CME: the date and time of first appearance in the LASCO/C2 field of view (FOV) and the central position angle (CPA). More than 10 CMEs can occur on a single day, and many CMEs can appear at the same time in the C2 FOV. The CPA can essentially distinguish these CMEs appearing simultaneously. CMEs an apparent width of 360 deg are marked as Halo in the CPA column. Halo CMEs can be symmetric (S) or asymmetric with respect to the occulting disk. Brightness asymmetry (BA) and outline asymmetry (OA). The halo CMEs are accordingly labeled as Halo (S), Halo (BA), and Halo (OA). The first column is linked to direct and difference LASCO/C2 movies with direct and difference EIT images superposed. These movies provide a complete view of the CME in question (including CMEs that cross day boundaries). The superposed EIT images, especially the difference images, are very useful in locating the solar sources of the CME (eruption region on the disk, at the limb or behind the limb).
Column 4 is the sky-plane width of CMEs, which is typically measured in the C2 FOV after the width becomes stable (early on, the width often increases). Information as to when the width was measured (#WDATA) is available in the text data containing original measurements as a sub-layer of column 2.
Each CME is characterized by three speeds: (1) the linear speed obtained by fitting a straight line (aka linear or first-order polynomial fit) to the height-time measurements, (2) quadratic speed obtained by fitting a parabola (aka quadratic or second-order polynomial fit) to the height-time measurements and evaluating the speed at the time of final (last possible) height measurement, and (3) speed obtained as in (2) but evaluated when the CME is at a height of 20 solar radii. Since the time of final height measurement varies from event to event, the 20 solar radii speed is useful for comparing different speeds. Caution must be exercised in dealing with CMEs that fade away before reaching 20 solar radii. For some CMEs, which show significant acceleration, the linear fit is not suitable. However, the linear speed serves as an average speed within the LASCO FOV. Clicking on any of the speeds displays the height-time plots with the fitted curves superposed. The actual height time measurements are also in the text file underlying the first-appearance column. It must be pointed out that the measurement is made at a single PA in 2-dimensional images. This means there is more information in the original data than presented in the catalog.
The acceleration of a CME can be positive, negative or close to zero meaning CMEs speed up, move with constant speed or slow down within the LASCO FOV. A minimum of three height-time measurements are needed for an estimate of the acceleration, but the accuracy increases when there are more measurements. Accelerations with just three measurements are not reliable and are marked with a superscript, *1.
Each CME is also characterized by a mass and a kinetic energy. There are generally large uncertainties in these numbers. Estimation of CME mass involves a number of assumptions, so the values given should be taken as representative. For example, most CMEs show an increase in mass when they traverse the first several solar radii, and then the mass reaches a quasi-constant value. This constant value is taken as the representative mass. Some CMEs fade within the first few solar radii. In these cases the mass corresponds to the time of last measurement. The mass estimates of halo CMEs are also very uncertain. The kinetic energy is obtained from the linear speed and the representative mass. Mass and kinetic energy values subject to such uncertainties are superscripted with *2.
The next column gives the position angle at which the height-time measurements are made (MPA for measurement position angle). Ideally, the MPA and CPA must be the same. However, some CMEs move nonradially so the two do not coincide. Even though there is no CPA for a halo CME, there is an MPA, corresponding to the PA of the fastest moving segment of the CME leading edge.
The last column of the monthly list contains some remarks regarding the number of data points and other limitations, as well as links to the halo CME alerts from the LASCO operator.
We regard the linear speed, width, CPA, and acceleration as the basic attributes of a CME. The text file linked to the first appearance time contains the actual height-time measurements, which may be useful for over plotting with other data. The text file also contains the CME onset times obtained by extrapolating the linear fit (#ONSET1) or quadratic fit (#ONSET2) to the solar surface (height = 1 solar radius). Note that theses extrapolations are accurate only for limb events. For disk events, the estimated onset is likely to be after the actual onset time. There is a quality index listed in the text file for each CME, on a scale of 1-5, 1 being poor and 5 being excellent.
This catalog can be searched from the VSO site (http://vso.nascom.nasa.gov/cgi-bin/vso/catalog.pl?) and from the catalog page (click on Catalog Search). The VSO site provides text outputs.
The catalog can also be searched by the SolarSoft routine ssw_getcme_list.pro running under an sswidl server. SSW users may use that function directly for application developement and to access this CME catalog within an SSW session (Author: Samuel Freeland, firstname.lastname@example.org). Optionally, output from that function may be input into ssw_cme2files.pro to map from events to email@example.com. The catalog search returns an html table similar to the monthly list in the catalog, but with a reduced set of parameters. It also includes a new feature, "Event Summary" which gives (1) the LASCO/C2 image of first CME appearance (difference image with EIT images superposed), (2) the GOES (1-8 A) soft X-ray profile with the time of the LASCO/C2 frame marked by a vertical line, (3) the height-time plot with linear fit, and (4) a simple html table giving the time of first appearance, Extrapolated onset time at 1 solar radius, number of CMEs in the range searched, search criteria used, and the number of CMEs meeting the search criteria.
Acknowledgement: Funding for the early phase of the catalog was provided by AFOSR and NSF. Currently, the catalog resides at the CDAW Data Center at Goddard Space Flight Center and is supported by NASA's Living with a Star program and the SOHO project.
There are several publications, which describe CMEs and qtheir properties. Interested readers should consult them:
S. Yashiro, G. Michalek, and N. Gopalswamy, A comparison of coronal mass ejections identified by manual and automatic methods, accepted for the publication in Annales Geophysicae
N. Gopalswamy, S. Yashiro, G. Michalek, G. Stenborg, A. Vourlidas, S. Freeland, and R. Howard, The SOHO/LASCO CME Catalog, accepted for the publication in Earth, Moon, and Planets
Gopalswamy, N., A. Lara, S. Yashiro, S. Nunes, and R. A. Howard, Coronal Mass Ejection Activity During Solar Cycle 23, In Solar variability as an input to the Earth's environment. Ed.: A. Wilson. ESA SP-535, Noordwijk: ESA Publications Division, p. 403, 2003
Gopalswamy, N., Nunes, S., Yashiro, S. and Howard, R. A., Variability of Solar Eruptions during cycle 23, Adv. Space Res., 34(2), 391, 2004.
Gopalswamy, N., A global picture of CMEs in the inner heliosphere, in The Sun and the Heliosphere as an Integrated System, edited by G. Poletto and S. T. Suess, Kluwer, Boston, Chapter 8, p. 201, 2004
St. Cyr, O. C. et al., Properties of coronal mass ejections: SOHO LASCO observations from January 1996 to June 1998, J. Geophys. Res., 105, 18169, 2000.
Vourlidas, A., Buzasi, D., Howard, R. A., and Esfandiari, E., Mass and energy properties of LASCO CMEs, Solar variability: from core to outer frontiers, Ed. A. Wilson. ESA SP-506, Vol. 1. Noordwijk: ESA Publications Division, p. 91, 2002
Yashiro, S., N. Gopalswamy, G. Michalek, O. C. St.Cyr, S. P. Plunkett, N. B. Rich, and R. A. Howard, A catalog of white light coronal mass ejections observed by the SOHO spacecraft, Journal of Geophysical Research, 109, A07105, doi:10.1029/2003JA010282, 2004.
(Author: N. Gopalswamy, 2005 September 30)