Seasonal trend of geomagnetic activity derived from solar-terrestrial geometry confirms an axial-equinoctial theory and reveals deficiency in planetary indices

At the magnetopause, solar wind plasma interacts with the terrestrial magnetic field, with the consequent entry of solar wind energy into the magnetosphere and the ionosphere. Geomagnetic activity is one of the results. Planetary geomagnetic indices, e.g. Kp, Ap, Am, etc, have been designed to measure solar particle radiation by its magnetic effects. Long-term averages of these indices have established that solar wind 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 o1' geomagnetic activity, its component parts, representing the axial and equinoctial hypotheses, have not so far been put together with a high degree of quantitative precision. This paper demonstrates that the semiannual trend of geomagnetic activity can be reproduced quantitatively with good 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 different months. Analysis shows that the seasonal trend of solar wind-magnetopause coupling is, in fact, controlled by a combination of the two competing theories, the axial and equinoctial, which have been advanced over the years to explain the semiannual variation in geomagnetic activity. Planetary ion density of the F2 layer of the ionosphere (F2pd) is another index of relatively higher accuracy which also shows marked maxima around the equinoxes. The observed seasonal trend of F2pd can be 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 planetary 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.