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.