Abstract
The existence of a 60–100 year periodic variation in solar and/or solar‐terrestrial phenomena has been a matter of dispute for many years. A wide variety of data sets previously have been analyzed, and the results of the analyses have been interpreted as showing evidence either for or against such a variation. However, all data sets that are proxy for solar wind in the ecliptic at 1 AU show variations consistent with a period of about 88 years. Here we report that a maximum entropy spectral analysis of the number of aurora reported per decade in Europe and the Orient from 450 A.D. to 1450 A.D. shows a strong stable line at a 88.4±0.7 years. Since the data set contains 11 cycles, this analysis establishes the reality of the “long cycle” for 1000 years. The mean amplitude and phase are then estimated from a superposed epoch analysis. The mean amplitude was 2.2 auroral reports per decade and the last minimum phase in these data occurred between 1403 A.D. and 1413 A.D.
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