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 f ield B-s near the equinoxes account for the observed higher geomagnetic act ivity in March and September. In a contrary point of view, we argue that th e bulk of the semiannual variation results from an equinoctial effect (base d 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 "mou ntain building" (creation of B-s) at the equinoxes but results primarily fr om "valley digging" (loss of coupling efficiency) at the solstices. We esti mate that this latter effect, which clearly reveals itself in the diurnal v ariation of the am index, is responsible for similar to 65% of the semiannu al modulation. The characteristic imprint of the equinoctial hypothesis is also apparent in hourly/monthly averages of the time-differentiated Dst ind ex and the AE index.