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

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 model based on the planetary tidal effect at the solar surface due to Mercury and Jupiter that, when combined with an equinoctial response of the magnetosphere, predicts the times of occurrence of predominantly quad-annual variation or predominantly 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-annual variation in the ~13.5 day component of aa index is predominant there is a large component at the 88 day periodicity of Mercury in the ~13.5 day component of solar wind speed. As further support for the model we establish that significant peaks in the aa index spectrum are due to an 88 day modulation of 27 day period solar activity. The model also predicts the occurrence of planetary tidal effect maximum in anti-phase with solar cycle maximums around 1970 and we show this is consistent with prior observations of higher solar emissions during the 1975 solar cycle minimum than in the following solar cycle maximum. This effect offers a possible explanation of the reduced solar cycle maximums around 1970 and during the Dalton Minimum.