Source Regions and Storm Effectiveness of Earth-Directed Halo CMEs Between 14 and 24 April 2002

All three halo CMEs occurred between 14 and 24 April 2002 are found to be associated with flares, interplanetary trasient shocks and geomagnetic storms, and thus are Earth-directed. To understand why the April 15 and 17 Earth-directed halo CMEs cause geomagnetic storms much stronger than the April 21 Earth-directed halo CME, we first determine the central position and the line-of-sight propagation velocity of the three halo CMEs using the cone model (Zhao et al., 2002), and then identify source regions of the halo CMEs and examine the magnetic configuration of those source regions. It is found that the April 15 and 17 halo CMEs originated in a bipolar closed field regions with the bipolar boundary layer being favourable for the occurence of the southward IMF event, and the April 21 halo CME originated in an unipolar region with the unipolar boundary layer being not favourable for the occurrence of the southward IMF event, though the later has a very high propagation velocity. The storm effectiveness of the three Earth-directed halo CMEs are discussed.