Kelvin-Helmholtz instability and the semiannual variation of geomagnetic activity

Kelvin-Helmholtz instability along the flanks of the magnetosphere exhibits a semiannual variation, instability maximums occurring at the equinoxes and instability minimums at the solstices. It is suggested that Kelvin-Helmholtz instabilities, generated at the magnetopause, initiate the modulation of geomagnetic disturbance detected as the semiannual variation of geomagnetic activity. The Kelvin-Helmholtz explanation predicts a universal time variation of geomagnetic disturbance. This prediction is confirmed by several analyses of the geomagnetic activity indices. Since the physical mechanism suggested for the semiannual variation of geomagnetic activity depends on the tilt of the dipole axis, the ‘equinoctial’ hypothesis is supported.