Why have geomagnetic storms been so weak during the recent solar minimum and the rising phase of cycle 24?

Abstract

The minimum following solar cycle 23 was the deepest and longest since the dawn of the space age. In this paper we examine geomagnetic activity using Dst and AE indices, interplanetary magnetic field (IMF) and plasma conditions, and the properties and occurrence rate of interplanetary coronal mass ejections (ICMEs) during two periods around the last two solar minima and rising phases (Period 1: 1995–1999 and Period 2: 2006–2012). The data is obtained from the 1-h OMNI database. Geomagnetic activity was considerably weaker during Period 2 than during Period 1, in particular in terms of Dst. We show that the responses of AE and Dst depend on whether it is solar wind speed or the southward IMF component (BS) that controls the variations in solar wind driving electric field (EY). We conclude that weak Dst activity during Period 2 was primarily a consequence of weak BS and presumably further weakened due to low solar wind densities. In contrast, solar wind speed did not show significant differences between our two study periods and the high-speed solar wind during Period 2 maintained AE activity despite of weak BS. The weakness of BS during Period 2 was attributed in particular to the lack of strong and long-duration ICMEs. We show that for our study periods there was a clear annual north–south IMF asymmetry, which affected in particular the intense Dst activity. This implies that the annual amount of intense Dst activity may rather be determined by the coincidence of what magnetic structure the strong ICMEs encountering the Earth have than by the solar cycle size.