IN 1984 a periodicity of 154 days was found in the record of solar flare activity1 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 flux1; hard X-ray emission2,3; H α flare activity4; microwave peak flux5; production of inter-planetary electrons6 and protons7; 10-cm radio flux8) as well as in sunspot area9,10. The cause of this 154-day periodicity, which has also been discovered in solar cycles 19 and 20 (refs 7, 11) remains unknown, but a suggestion that it was related to enhanced flare activity in certain longitude bands has been ruled out3. 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.
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Bai, T., Sturrock, P. The 154-day and related periodicities of solar activity as subharmonics of a fundamental period. Nature 350, 141–143 (1991) doi:10.1038/350141a0
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