A Statistical Study of Two Classes of Coronal Mass Ejections

A comprehensive statistical study is performed to address the question of whether two classes of coronal mass ejections (CMEs) exist. A total of 3217 CME events observed by SOHO/LASCO in 1996-2000 have been analyzed. We have examined the distributions of CMEs according to speed and acceleration, respectively, and investigated the correlation between speed and acceleration of CMEs. This statistical analysis is conducted for two subsets containing those CMEs that show a temporal and spatial association either with GOES X-ray solar flares or with eruptive filaments. We have found that CMEs associated with flares have a higher median speed than those associated with eruptive filaments and that the median speed of CMEs associated with strong flares is higher than that of weak-flare-associated CMEs. The distribution of CME acceleration shows a conspicuous peak near zero, not only for the whole data set, but also for the two subsets associated either with solar flares or with eruptive filaments. However, we have confirmed that the CMEs associated with major flares tend to be more decelerated than the CMEs related to eruptive filaments. The fraction of flare-associated CMEs has a tendency to increase with the CME speed, whereas the fraction of eruptive-filament-associated CMEs tends to decrease with the CME speed. This result supports the concept of two CME classes. We have found a possibility of two components in the CME speed distribution for both the CME data associated with flares larger than M1 class and the CME data related with limb flares. Our results suggest that the apparent single-peak distribution of CME speed can be attributed to the projection effect and possibly to abundance of small flares too. We also note that there is a possible correlation between the speed of CMEs and the time-integrated X-ray flux of the CME-associated limb flares.