LAUSR

Abstract. Seasonal features of geomagnetic activity and their solar wind-interplanetary drivers are studied using more than 5 solar cycles of geomagnetic activity and solar wind observations. This study involves a total of 1239 geomagnetic storms of varying intensity identified using the Dst index from January 1963 to December 2019, a total of 75863 substorms identified from the SML index from January 1976 to December 2019, a total of 145 high-intensity long-duration continuous auroral electrojet (AE) activity (HILDCAA) events identified using the AE index from January 1975 to December 2017. The occurrence rates of the substorms, geomagnetic storms, including moderate (−50 nT ≥ Dst > −100 nT) and intense (−100 nT ≥ Dst > −250 nT), exhibit a significant semi-annual variation (periodicity ~ 6 months), while the super storms (Dst ≤ −250 nT) and HILDCAAs do not exhibit any clear seasonal feature. The geomagnetic activity indices Dst and ap exhibit a semi-annual variation while AE exhibits an annual variation (periodicity ~ 1 year). The annual and semi-annual variations are found to be driven by the annual variation of the solar wind speed Vsw and the semi-annual variation of the coupling function V Bs (where V = Vsw, and Bs is the southward component of the interplanetary magnetic field), respectively. We present a detailed analysis of the annual and semi-annual variations and their dependencies on the solar activity cycles separated as the odd, even, weak and strong solar cycles.