Abstract
A total of 1,267 aurorae, from the Yerkes Observatory records, covering a 55-year interval, have been analyzed. The number per year clearly shows an 11-year cycle, having minima that coincide with minimum solar activity. The maximum of auroral activity, however, follows the maximum of solar activity by approximately two years. A harmonic analysis, extending from 1 to 400 days, was made to examine the data for recurrence tendencies. After correction for a large-amplitude 29.5-day periodicity, due to observational interference from moonlight, a definite but very small recurrence tendency is found at 27 days.
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