The 154-day and related periodicities of solar activity as subharmonics of a fundamental period

In 1984 a periodicity of 154 days was found in the record of solar flare activity from 1980 to 1983. Since then, the same periodicity has been found in many different measures of solar flare activity during cycle 21 (soft X-ray peak flux; hard X-ray emission, Hα flare activity, microwave peak flux; production of interplanetary electrons and protons; 10-cm radio flux) as well as in sunspot area. The cause of this 154-day periodicity, which has also been discovered in solar cycles 19 and 20 remains unknown, but a suggestion that it was related to enhanced flare activity in certain longitude bands has been ruled out. Here we show that periodicities of 51, 78, 104 and 129 days, in addition to the 154-day period, can often be detected in flare and sunspot records. These periods are close to integral multiples (by factors of 2, 3, 4, 5 and 6) of 25.8 days, suggesting that they are subharmonics of a fundamental period. (author)