Evolution of geomagnetic <i>aa</i> index near sunspot minimum

Abstract

The smoothed values of the minima of sunspot number Rz and the geomagnetic index aa were compared for sunspot cycles 12–23. In one cycle, aa(min) occurred earlier than Rz(min), but remained at that low from a few months before Rz(min) to a few months after Rz(min). In two cycles, Rz(min) and aa(min) coincided within a month or two. In nine cycles, aa(min) occurred more than three months later than Rz(min). The aa(min) coincided with the minima of some solar radio emission indices originating in the solar corona. For sunspot cycles 21, 22, 23, the minimum of solar wind velocity V occurred 0–9 months later than the aa(min). The minimum of solar wind total magnetic field B occurred near Rz(min). The solar wind ion density N had maxima (instead of minima) near Rz(min), and again near Rz(max), indicating a  ~5-year periodicity, instead of an 11-year periodicity. The maxima of aa, V and B occurred near Rz(max) and/or later in the declining phase of Rz. The aa index was very well correlated with the functions BV and BV 2.Key words. Geomagnetism and paleomagnetism (time variations, diurnal to secular – time variations, secular and long term) Interplanetary physics (interplanetary magnetic field)