A 12‐month wave in geomagnetic activity

Abstract

Besides the well‐known 6‐month wave in geomagnetic activity and its recurrence tendency, no persistent 12‐month wave has been detected by harmonic analysis of consecutive monthly values. Such a 12‐month wave, however, can be shown to exist if, instead, single activity sequences are analyzed, i.e., daily values at intervals of one solar rotation. A harmonic sequence analysis of this kind has been carried out for the daily character figures C8 from 1884 to 1968. For years before and including sunspot minimums two clearly distinguished significant 12‐month waves having nearly equal amplitudes and opposite phases are obtained. The maximum occurs shortly after one of the two equinoxes: at the end of March or at the beginning of October. The amplitude is almost 3 times that of the semiannual wave in the same sequences. For years around sunspot maximums the amplitude is smaller, and in the harmonic dial the clouds of points for both 12‐month waves can no longer be clearly distinguished. The dates of maximum of the revealed 12‐month wave again exclude an explanation by means of the classical ‘axial hypothesis.’ The statistical result instead suggests an interpretation primarily by means of a temporary N‐S asymmetry in the distribution of the magnetic M regions within single solar sectors and the same magnetospherical modulation mechanism responsible for the 6‐month wave.