The variance (power) spectra ofCi, Kp, andAp

Abstract

Time series of the geomagnetic disturbance indices Ci, Kp, and Ap, are subjected to variance (power) spectrum analysis. Application of this technique yields the distribution of the statistical variance of a time series as a function of the frequency or period of the variations. Furthermore, its use overcomes certain theoretical and computational difficulties inherent in standard Fourier analysis. The spectra of the three indices are identical in almost all respects. They demonstrate the mathematical existence of the 6-month period, and periods of 27, 14, 9, 7, 5½, and 4½ (27/N, N = 1,2, 3, 4, 5, and 6) days. No interpretation of the overtones of the 27-day period is given. The spectra show that, over intervals of some months, there are considerable changes in the distribution of the variance among these periods. However, there are few systematic differences in the spectra between times of sunspot maxima and minima.