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Ann. Geophys., 20, 1519-1527, 2002
www.ann-geophys.net/20/1519/2002/
© European Geosciences Union 2002
Evolution of geomagnetic aa index near sunspot minimum
R. P. Kane
Instituto Nacional de Pesquisas Espacias, C. P. 515, São Jose dos Campos, 12201-970, SP, Brazil
Correspondence to: R. P. Kane (kane@laser.inpe.br)
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)
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