A SEASONAL DEPENDENCE FOR THE GEOEFFECTIVENESS OF ERUPTIVE SOLAR EVENTS

The pronounced seasonal variation of the occurrence rate of great geom agnetic storms, attributed to in-ecliptic draping of interplanetary ma gnetic fields and the Russell-McPherron effect, suggests that the geoe ffectiveness of the causative eruptive solar events has a seasonal dep endence. Thus an eruptive solar event of a given 'size' occurring near the equinoxes might be expected to give rise to a larger geomagnetic storm than would a comparable event occurring near the solstices. We p resent the following evidence for such a seasonal dependence: (1) the great 'problem' storms of the last four solar cycles, i.e., severe sto rms lacking commensurate preceding solar activity, occurred relatively near the equinoxes, (2) the few great storms that occurred near the s olstices were generally preceded by truly outstanding flares, and (3) on average, central meridian proton flares occurring near the equinoxe s were followed by significantly larger geomagnetic storms than were s imilar flares occurring near the solstices. We conclude that the stron g semi-annual variation of great storms results from the virtual absen ce, near the solstices, of great storms associated with disappearing s olar filaments and with moderately-sized eruptive solar flares.