Properties of coronal mass ejections: SOHO LASCO observations from January 1996 to June 1998

Abstract

We report the properties of all the 841 coronal mass ejections (CMEs) observed by the Solar and Heliospheric Observatory (SOHO) Large Angle Spectroscopic Coronagraph (LASCO) C2 and C3 white‐light coronagraphs from January 1996 through June 1998, and we compare those properties to previous observations by other similar instruments. Both the CME rate and the distribution of apparent locations of CMEs varied during this period as expected based on previous solar cycles. The distribution of apparent speeds and the fraction of CMEs showing acceleration were also in agreement with earlier reports. The pointing stability provided by an L‐1 orbit and the use of CCD detectors have resulted in superior brightness sensitivity for LASCO over earlier coronagraphs; however, we have not detected a significant population of fainter (i.e., low mass) CMEs. The general shape of the distribution of apparent sizes for LASCO CMEs is similar to those of earlier reports, but the average (median) apparent size of 72° (50°) is significantly larger. The larger average apparent size is predominantly the result of the detection of a population of partial and complete halo CMEs, at least some of which appear to be events with a significant longitudinal component directed along the Sun‐Earth line, either toward or away from the Earth. Using full disk solar images obtained by the Extreme ultraviolet Imaging Telescope (EIT) on SOHO, we found that 40 out of 92 of these events might have been directed toward the Earth, and we compared the timing of those with the Kp geomagnetic storm index in the days following the CME. Although the “false alarm” rate was high, we found that 15 out of 21 (71%) of the Kp ≥ 6 storms could be accounted for as SOHO LASCO/EIT frontside halo CMEs. If we eliminate three Kp storms that occurred following LASCO/EIT data gaps, then the possible association rate was 15 out of 18 (83%).