A SEASONAL DEPENDENCE FOR THE GEOEFFECTIVENESS OF ERUPTIVE SOLAR EVENTS

The pronounced seasonal variation of the occurrence rate of great geomagnetic storms, attributed to in-ecliptic draping of interplanetary magnetic fields and the Russell-McPherron effect, suggests that the geoeffectiveness of the causative eruptive solar events has a seasonal dependence. 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 present the following evidence for such a seasonal dependence: (1) the great 'problem' storms of the last four solar cycles, i.e., severe storms lacking commensurate preceding solar activity, occurred relatively near the equinoxes, (2) the few great storms that occurred near the solstices were generally preceded by truly outstanding flares, and (3) on average, central meridian proton flares occurring near the equinoxes were followed by significantly larger geomagnetic storms than were similar flares occurring near the solstices. We conclude that the strong semi-annual variation of great storms results from the virtual absence, near the solstices, of great storms associated with disappearing solar filaments and with moderately-sized eruptive solar flares.