Long-term north-south asymmetry in solar wind speed inferred from geomagnetic activity: A new type of century-scale solar oscillation?

A significant and very similar annual variation in solar wind speed and in geomagnetic activity was recently found around all the four solar cycle min ima covered by direct SW observations since mid-1960's. We have shown that the phase of this annual variation reverses with the Sun's polarity reversa l, depicting a new form of 22-year periodicity. The annual variation result s from a small north-south asymmetry in SW speed distribution where the min imum speed region is shifted toward the northern magnetic hemisphere. Here we study the very long-term evolution of the annual variation using early r egistrations of geomagnetic activity. We find a significant annual variatio n during the high-activity solar cycles in mid-19th century and since 1930' s. Most interestingly, the SW speed asymmetry in mid-19th century was oppos ite to the present asymmetry, i.e., the minimum speed region was then shift ed toward the southern magnetic hemisphere. This change of asymmetry sugges ts for a possible new form of century-scale oscillation in the north-south asymmetry of the Sun. We explain the asymmetry in terms of a relic magnetic field dislocated slightly in the north-south direction from the heliograph ic equator. The change in the asymmetry would result from the century-scale north-south oscillation of the location of the relic field across the ecli ptic.