Hemispheric asymmetry of the Sun suggested by the annual variation ofthe aa index.

The annual variation of Mayaud's (1973) aa index has been discovered to exhibit unequal spring and fall maxima, the relative dominance of the two equinoxes varying in a quasi-periodic way. This finding suggests that one magnetic hemisphere of the sun might predominate slightly over the other for several years in succession, the dominance then switching over in a quasi-periodic way. The result of this magnetic asymmetry of the sun would be a droop of the current sheet in the solar wind, and a difference of the solar magnetic latitudes of the earth in the two equinoxes, resulting in an equinoctial asymmetry in the energy transfer from the solar wind into the magnetosphere and consequently, in geomagnetic agitation. Comparison with other available pieces of evidence suggests that some nonreconnection mechanism, such as viscous interaction at the flanks of the magnetosphere, might play an important role in geomagnetic agitation.