Periodic variations of geomagnetic activity indices

Abstract

Geomagnetic activity is generated by the interaction of the solar wind with the magnetosphere in connection with energy and mass transfer. Most observations suggest that the solar wind energy is transferred to the magnetosphere by the reconnection process at the dayside magnetopause, where oppositely directed southward IMF and northward magnetospheric magnetic field lines merge. Yearly averages of geomagnetic activity indices Ap for the years 1965 to 2009 are compared to the respective averages of v and Bx, By, Bz, where v is the solar wind velocity and Bx, By, Bz are the southward, northward and north-south component of inter- planetary magnetic field (IMF). The correlation of these quantities is known to be rather good. Comparing the averages of Ap with v and Bx, By, Bz separately we find that, during the declining phase of the solar cycle, v and during the ascending phase Bx have more influence on Ap. Analysing the daily variation of ap in an analogue manner suggesting that during phases of the solar cycle, when ap depends more on the IMF- Bx, By, Bz, components, whereas during phases when ap increases as v increases.