Properties of coronal mass ejections: SOHO LASCO observations from January1996 to June 1998

We report the properties of all the 841 coronal mass ejections (CMEs) obser ved by the Solar and Heliospheric Observatory (SOHO) Large Angle Spectrosco pic Coronagraph (LASCO) C2 and C3 white-light coronagraphs from January 199 6 through June 1998, and we compare those properties to previous observatio ns by other similar instruments. Both the CME rate and the distribution of apparent locations of CMEs varied during this period as expected based on p revious solar cycles. The distribution of apparent speeds and the fraction of CMEs showing acceleration were also in agreement with earlier reports. T he pointing stability provided by an L-l orbit and the use of CCD detectors have resulted in superior brightness sensitivity for LASCO over earlier co ronagraphs; however, we have not detected a significant population of faint er (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 avera ge (median) apparent size of 72 degrees (50 degrees) is significantly large r. The larger average apparent size is predominantly the result of the dete ction of a population of partial and complete halo CMEs at least some of wh ich 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 (EI T) on SOHO, we found that 40 out of 92 of these events might have been dire cted toward the Earth, and we compared the timing of those with the I(I) ge omagnetic storm index in the days following the CME, Although the "false al arm" rate was high, we found that 15 out of 21 (71%) of the K-p greater tha n or equal to 6 storms could be accounted for as SOHO LASCO/EIT frontside h alo CMEs. If we eliminate three Kp, storms that occurred following LASCO/EI T data gaps. then the possible association rate was 15 out of 18 (83%).