Spectral analysis of sunspot number and geomagnetic indices (1868–2001)

Abstract

The multiple-taper spectral analysis was applied to a a, A p and R z annual and monthly average series (1868–2001). The significant periods at 95% confidence level for annual averages are 11.1 and 10.2 years for R z and 11.1, 5.3, 4.3, 2.7 and 2.1 years for a a. For monthly averages, the significant periods were 133 (11.1 years) and 17.7 months for R z and 133 (11.1 years), 63.7 (5.3 years), 17.5, 12.7 and 6 months for a a. The periods associated with the solar cycle are observed in both series. The a a spectrum showed in addition a peak near 4.3 years, which is not present in the R z spectrum. Other works have also shown this peak in geomagnetic activity. This peak is believed to be caused by the dual-peak structure in the a a. The 22-year Hale cycle was not detected either in R z or a a spectrum in the period 1868–2001, but it was seen in R z data for 1700–2000 annual averages. The explanation of this 22-year cycle has been questioned and while in some spectral analysis this periodicity has been found, others have failed in its detection. The a a spectrum showed in addition a peak of 6 months, associated to the semi-annual variation of geomagnetic activity. Three hypotheses have been considered to explain the seasonal activity variation. These are the Russell–McPherron effect, the equinoctial effect and the axial effect.