Mountains versus valleys: Semiannual variation of geomagnetic activity

The semiannual variation in geomagnetic activity is generally attributed to the Russell-McPherron effect. In that picture, enhancements of southward field B-s near the equinoxes account for the observed higher geomagnetic activity in March and September. In a contrary point of view, we argue that the bulk of the semiannual variation results from an equinoctial effect (based on the psi angle between the solar wind flow direction and Earth's dipole axis) that makes B-s coupling less effective (by similar to 25% on average) at the solstices. Thus the semiannual variation is not simply due to mountain building (creation of B-s) at the equinoxes but results primarily from valley digging (loss of coupling efficiency) at the solstices. We estimate that this latter effect, which clearly reveals itself in the diurnal variation of the am index, is responsible for similar to 65% of the semiannual modulation. The characteristic imprint of the equinoctial hypothesis is also apparent in hourly/monthly averages of the time-differentiated Dst index and the AE index.