Abstract
From maximum entropy spectral analysis (MESA) of short lengths ofAa indices of geomagnetic activity, the characteristics of the strength of the 27-day and 13.5-day signals for each of the solar cycles 11 to 21 are highlighted. It is shown that the 13.5-day signal is a near permanent feature in geomagnetic activity. The Hale-cycle (22 yr) effect could be seen in the average magnitude of the 27-day signal with greater strengths in the even cycles. No clear annual variation in the strength is noticed, contrary to some earlier known results.
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