New Geoeffective Parameters of Very Fast Halo Coronal Mass

We have examined the physical characteristics of very fast coronal mass ejections (CMEs) and their geoeffective parameters. For this we consider SOHO/LASCO CMEs whose speeds are larger than 1300 km s-1. By examining all SOHO/EIT and SOHO/LASCO images of the CMEs, we selected 38 front-side very fast CMEs and then examined their associations with solar activity such as X-ray flares and Type II bursts. As a result, we found that among these frontside fast CMEs, 25 are halo (or full halo) CMEs with span of 360 degrees; 12 are partial halo CMEs with span greater than 130 degrees; only 1 is broadside CME with span of 53 degrees. There are 13 events that are shock deflected CMEs: 6 are full halo CMEs and 7 are partial halo CMEs. It is found that about 60 % (23/38) CMEs were ejected from the western hemisphere. We also note that these very fast CMEs have very high associations with other solar activities: all the CMEs are associated with X-ray flares (X-12, M-23, C-3), and about 80 % of the CMEs (33/38) were accompanied by Type II bursts. For the examination of CME geoeffectiveness, we select 12 halo CMEs whose longitudes are less than 40 degrees, which are thought to be the most plausible candidates of geoeffective CMEs. Then we examine the relationship between their CME physical parameters (mass, column density, location of an associated flare, and direction) and the Dst index. Especially, a CME direction parameter, which is defined as the maximum ratio of its shorter front from solar disk center and its longer one, is proposed as a new geoeffective parameter. Its major advantage is that it can be directly estimated from coronagraph observation. It is found that while the location of the associated flare has a poor relationship with the Dst index, the new direction parameter has a relatively good relationship. In addition, the column density of a CME also has a comparable good correlation with the Dst index. Noting that the CME column density is strongly affected by the direction of a CME, our results imply that the CME direction seems to be the most important parameter that controls the geoeffectiveness of very fast halo CMEs.