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[92.11] Evidence for a 25.5-day Period as a Fundamental Period

T. Bai, T.A. Bai (Stanford University)

Occurrence of major flares episodically exhibits mid-term periodicities in the range from two to several solar rotations. Frequently observed periods are 51 days, 77 days, 127 days, and 153 days, which are integral multiples of 25.5 days. For this reason, Bai and Sturrock (1991) proposed 25.5 days as a fundamental period. By annalyzing the power spectrum of sunspot area data collected during more than 10 solar cycles, I find a strong peak at 13.7 days, which is one half of 27.4 days. This period corresponds to a synodic period of a sidereal period of 25.5 day. The fact that this period (25.5 days) is much more persistent than episodically observed mid-term periodicities is consistent with the idea that it is a fundamental period.

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