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We show that there is a previously unreported quad-annual variation in the~13.5 day component of the aa index of geomagnetic activity. We derive a modelbased on the planetary tidal effect at the solar surface due to Mercury andJupiter that, when combined with an equinoctial response of the magnetosphere,predicts the times of occurrence of predominantly quad-annual variation orpredominantly semi-annual variation in the ~13.5 day component of the aa index.In support of the model we show that, during years when the quad-annualvariation in the ~13.5 day component of aa index is predominant there is alarge component at the 88 day periodicity of Mercury in the ~13.5 day componentof solar wind speed. As further support for the model we establish thatsignificant peaks in the aa index spectrum are due to an 88 day modulation of27 day period solar activity. The model also predicts the occurrence ofplanetary tidal effect maximum in anti-phase with solar cycle maximums around1970 and we show this is consistent with prior observations of higher solaremissions during the 1975 solar cycle minimum than in the following solar cyclemaximum. This effect offers a possible explanation of the reduced solar cyclemaximums around 1970 and during the Dalton Minimum.

Association of a planetary tidal effect with the time variation of the ~13.5 day component of geomagnetic activity