On the Relationship between Solar Wind Characteristics and Geomagnetic Activity

A number of geomagnetic indices have been introduced to describe the variation of the geomagnetic field as a response to the variable conditions in the solar wind. The most important imprint on the terrestrial magnetosphere belongs to the high speed streams in solar wind which are transferring part of their energy to the magnetosphere. The paper analyses the aa geomagnetic index during the 1964-2007 period that includes four 11-yr solar cycles (nos. 20-23) and two magnetic solar cycles (Hale cycle or 22-yr solar cycle). The 11-yr solar cycles, analyzed by the monthly values of the sunspot relative number (Wolf number), are quite different by their intensity and durations. The pairs of two such cycles (nos. 20 and 21, nos. 22 and 23, respectively) constitute 22-yr solar magnetic cycles. The peculiarities of the aa series are detailed in comparison with 11-yr solar cycle. The aa series present a more complex variability during the analysed period, with more maxima. Some of these maxima are found nearby the sunspot maxima of the 11-yr solar cycles but the most important of them are placed on the descendant phase of the solar cycle. The last ones could be explained as consequences of the variability registered by the high speed streams in the solar wind during the corresponding phase of solar cycle. The dynamics of the geomagnetic aa data series and some solar wind parameter data series was analysed using the correlation and spectral analysis.