On the periodic variations of geomagnetic activity indices Ap and ap

Yearly averages of geomagnetic activity indices Ap for the years 1967±1984 are compared to the respective averages of m Bs, where v is the solar wind velocity and Bs is the southward interplanetary magnetic ®eld (IMF) component. The correlation of both quantities is known to be rather good. Comparing the averages of Ap with m and Bs separately we ®nd that, during the declining phase of the solar cycle, m and during the ascending phase Bs have more in ̄uence on Ap. According to this observation (using Fourier spectral analysis) the semiannual and 27 days, Ap variations for the years 1932±1993 were analysed separately for years before and after sunspot minima. Only those time-intervals before sunspot minima with a signi®cant 27-day recurrent period of the IMF sector structure and those intervals after sunspot minima with a signi®cant 28±28.5-day recurrent period of the sector structure were used. The averaged spectra of the two Ap data sets clearly show a period of 27 days before and a period of 28±29 days after sunspot minimum. Moreover, the phase of the average semiannual wave of Ap is signi®cantly di€erent for the two groups of data: the Ap variation maximizes near the equinoxes during the declining phase of the sunspot cycle and near the beginning of April and October during the ascending phase of the sunspot cycle, as predicted by the RussellMcPherron (R-M) mechanism. Analysing the daily variation of ap in an analogue manner, the same equinoctial and R-M mechanisms are seen, suggesting that during phases of the solar cycle, when ap depends more on the IMF-Bs component, the R-M mechanism is predominant, whereas during phases when ap increases as v increases the equinoctial mechanism is more likely to be e€ective.