Coronal mass ejections, interplanetary ejecta and geomagnetic storms

Studies using SOHO spacecraft data have demonstrated that frontside halo coronal mass ejections (CMEs) detected by the LASCO coronagraphs generally precede geomagnetic storms. Nonetheless, about three quarters of such CMEs do not result in even moderate geomagnetic activity. We study the relationship of all the ejecta (interplanetary CMEs) which passed Earth during 1996-1999 to coronagraph CMEs and geomagnetic activity. We reach the following conclusions: (1) Only about half of frontside halo CMEs encounter the Earth; (2) The geoeffectiveness of ejecta depends strongly on the southward magnetic field strength and, for the same southward field, is irrespective of whether or not the ejecta has a magnetic cloud structure; (3) Transit speeds of ejecta to Earth are only loosely correlated with CME speeds, one influence being the prevailing solar wind conditions between the Sun and Earth; (4) Ejecta may be detected at Earth even when there is no preceding halo CME observed by LASCO. Such ejecta are not particularly geoeffective.