Spectral analysis results for sudden storm commencements 1868–1996 Original Research Article

We perform a power spectrum analysis of the occurrence of sudden storm commencements for solar cycles 11 to 22. Studying in particular peaks longer than the 11 years solar cycle, we found a significant periodicity at around 20 to 30 years, which seems to be unstable. Similar three-cycle unstable peaks are observed in other solar and geomagnetic phenomena such as sunspots, auroral occurrence and the Ap index. The origin of such a periodicity in the occurrence of storm commencements is unknown at present. Due to its unstable behaviour it is not possible to predict when it will appear in the SSCs data.