Halo Coronal Mass Ejections and their Geoeffectiveness

N. Gopalswamy

NASA Goddard Space Flight Center

Halo coronal mass ejections (CMEs), once thought to be a rare novelty, are routinely observed by SOHO coronagraphs. Halo CMEs been found to be an important players in the physics of Sun-Earth connection in causing geomagetic storms and accelerating energetic particles. In this talk, I discuss the geoeffectiveness, speed, solar source, and flare association of a set of 378 halo coronal mass ejections (CMEs) of cycle 23. I compare the distributions of Dst values for the following subsets of halo CMEs: disk halos (within 45 deg from disk center), limb halos (beyond 45 degrees but within 90 deg from disk center), and backside halo CMEs. On the average, disk halos are followed by intense storms, limb halos are followed by moderate storms, and backside halos are not followed by significant storms. The Dst distribution for a random sample is nearly identical to the case of backside halos. About 71% of all frontside halos are geoeffective, supporting the high rate of geoeffectiveness of halo CMEs. A larger fraction of disk halos is geoeffective. The geoeffectiveness rate has prominent dips in 1999 and 2002 (the beginning and end years of the solar maximum phase). The number of geoeffective halos shows a triple peak as the number of intense geomagnetic storms does. Intense storms generally are due to disk halos and the few intense storms from limb halos occurred only in the maximum and declining phases. Most intense storms occur when there are successive CMEs. The difference in flare sizes among geoeffective and non-geoeffective halos is not significant. The non-geoeffective CMEs are generally slower and have more easterly or limbward solar sources compared to the geoeffective ones, source location and speed are the most important parameters for geoeffectiveness.