In this paper a spectral analysis of the aa geomagnetic<br>index (aa) and of the sunspot number (SSN) time<br>series during the period 1868-1998 is made. The<br>spectral method used was the multiple taper method.<br>The main periods found were related to the 11 year<br>solar cycle and its harmonics. The aa spectrum shows<br>in addition a period of 4.4 years, which is caused by<br>the dual-peak distribution of the aa time series. The<br>cross-correlation for all the periods indicates that aa<br>lags SSN by 1 year, however the correlation is<br>decreasing and the lag is increasing with time. Using<br>averages of 23 years, cross-correlations calculations<br>show that the correlation coefficient decreases from<br>0.76 to 0.35 and the lag from 0 (1868-1890) to –3 (1960-1982).


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