SOLAR-WIND AND EQUINOCTIAL MAXIMA IN GEOPHYSICAL PHENOMENA

Several geophysical phenomena, e.g., geomagnetic activity, auroral dis plays, thermospheric neutral gas density and temperature, F2 layer ion density of the ionosphere, intensity of the ring current, etc., are k nown to exhibit a semiannual variation which shows prominent maxima ar ound the equinoctial months. While the solar wind is now generally acc epted as the cause for the occurrence of equinoctial maxima in geomagn etic activity and aurora, the basic reason for the occurrence of such a feature in the F2 layer ion density and others is not presently well understood. We present tangible evidence and analysis in this paper t o suggest that the solar wind could be the driving force behind the oc currence of prominent equinoctial maxima in a geophysical phenomenon w hich cannot be explained on the basis of the seasonal variation of sol ar declination or solar zenith angle. We show that the seasonal enhanc ement factors (SEF) of the planetary ion density of the F2 layer are a characteristic signature of the entry of solar wind energy into geosp ace which can be used to demonstrate quantitatively that the solar win d could be the basic cause of the prominent equinoctial maxima which o ccur in several geophysical phenomena. (C) 1998 Elsevier Science Ltd. All rights reserved.