Evolution of geomagnetic aa index near sunspot minimum

Other literature type English OPEN
Kane, R. P. (2002)

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