Semiannual variation of the geomagnetic Dst index:: Evidence for a dominant nonstorm component

The geomagnetic Dst index exhibits a strong semiannual variation with amplitude of 5.3 nT (oil an average baseline of -16.4 nT) for 1957-1997. If we consider the five quietest Dst days for each month during this interval. we find that while the average Dst baseline shifts froth -16.4 to +4.0 nT, the amplitude of the 6-month wave remains relatively strong at 2.5 nT. Thus similar to 50% (2.5 nT/5.3 nT) of the seasonal variation of the Dst index results froth modulation of its quiet time baseline. The seasonal modulation of Dst appears to consist of both a storm component, resulting from rapid variations of the ring current and other currents, and a slowly varying. nonstorm component. We estimate that the storm component accounts for only similar to 30-50% of the amplitude of the 6-month wave ill Dst versus similar to 50-70% for the nonstorm component. The classic equinoctial effect appears to dominate the storm component. accounting for 20-40% of the amplitude of the 6-month wave ill Dst versus similar to 10% for the combined axial/Russell-McPherron mechanisms. Candidate mechanisms for the nonstorm component of the 6-month wave in Dst include the Malin-Isikara effect (the seasonal displacement. of ring/tail currents by solar wind compression) and a semiannual variation of magnetopause currents.