Power spectral analysis of auroral occurrence frequency

Abstract

Power spectral analyses have been carried out on a data set of Swedish visual auroras for the period 1721 to 1943. Besides the anticipated 11‐year, annual, and semiannual cycles, an unexpected peak near 1.4 years is found. The amplitude of this latter peak, while smaller than that of the 6‐month and 12‐month peaks, is statistically significant at between the 5% and 1% level and is found consistently in a sequence of spectra covering consecutive, overlapping, approximately 22‐year data segments. The semiannual variation becomes extremely weak during the prolonged solar activity minimum at the beginning of the nineteenth century, and also shows some lessening during the prolonged solar activity minimum near the beginning of the twentieth century. The 1.4‐year peak shows a strong modulation of about 65 to 68 years and an additional long‐term trend of the order of centuries. The origin of this peak is unknown.