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 (on 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 from −16.4 to +4.0 nT, the amplitude of the 6‐month wave remains relatively strong at 2.5 nT. Thus ∼50% (2.5 nT/5.3 nT) of the seasonal variation of the Dst index results from 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 ∼30–50% of the amplitude of the 6‐month wave in Dst versus ∼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 in Dst versps ∼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.