SEASONAL TREND OF GEOMAGNETIC-ACTIVITY DERIVED FROM SOLAR-TERRESTRIALGEOMETRY CONFIRMS AN AXIAL-EQUINOCTIAL THEORY AND REVEALS DEFICIENCY IN PLANETARY INDEXES

At the magnetopause, solar wind plasma interacts with the terrestrial magnetic field, with the consequent entry of solar wind energy into th e magnetosphere and the ionosphere. Geomagnetic activity is one of the results. Planetary geomagnetic indices, e.g. Kp, Ap, Am, etc, have be en designed to measure solar particle radiation by its magnetic effect s. Long-term averages of these indices have established that solar win d energy input into the ionosphere maximizes around equinoctial months with minima around the solstices. Although considerable progress has been made to explain qualitatively the semiannual variation of geomagn etic activity, its component parts, representing the axial and equinoc tial hypotheses, have not so far been put together with a high degree pf quantitative precision. This paper demonstrates that the semiannual trend of geomagnetic activity can be reproduced quantitatively with g ood precision by using accurate astronomical data relating to the Sun- Earth geometry. The key factor is the combination of the varying solar declination and the heliographic latitude of the Earth during differe nt months. Analysis shows that the seasonal trend of solar wind-magnet opause coupling is, in fact, controlled by a combination of the two co mpeting theories, the axial and equinoctial, which have been advanced over the years to explain the semiannual variation in geomagnetic acti vity. Planetary ion density of the F2 layer of the ionosphere (F2pd) i s another index of relatively higher accuracy which also shows marked maxima around the equinoxes. The observed seasonal trend of F2pd can b e reproduced by using the semiannual trend of geomagnetic activity as derived from astronomical data with a correlation coefficient of 0.98. This analysis also brings out another important fact that the planeta ry indices, Kp, Ap, Am and AA, are somewhat deficient as they respond to solar declination only and do not bring out the contribution of the heliographic latitude of the Earth. (C) 1996 Elsevier Science Ltd