Several geophysical phenomena\ e[g[\ geomagnetic activity\ auroral displays\ thermospheric neutral gas density and temperature\ F1 layer ion density of the ionosphere\ intensity of the ring current\ etc[\ are known to exhibit a semiannual variation which shows prominent maxima around the equinoctial months[ While the solar wind is now generally accepted as the cause for the occurrence of equinoctial maxima in geomagnetic activity and aurora\ the basic reason for the occurrence of such a feature in the F1 layer ion density and others is not presently well understood[ We present tangible evidence and analysis in this paper to suggest that the solar wind could be the driving force behind the occurrence of prominent equinoctial maxima in a geophysical phenomenon which cannot be explained on the basis of the seasonal variation of solar declination or solar zenith angle[ We show that the seasonal enhancement factors "SEF# of the planetary ion density of the F1 layer are a characteristic signature of the entry of solar wind energy into geospace which can be used to demonstrate quantitatively that the solar wind could be the basic cause of the prominent equinoctial maxima which occur in several geophysical phenomena[ Γ 0887 Elsevier Science Ltd[ All rights reserved[