Nonideal high-β magnetohydrodynamic Kelvin-Helmholtz instability driven by the shear in the ion diamagnetic drift velocity at the subsolar magnetopause

Abstract

A nonideal magnetohydrodynamic (MHD) Kelvin-Helmholtz (K-H) instability peculiar to a high-β plasma with a nonuniform pressure is studied for the magnetosheath field due north at the subsolar magnetopause, where the ideal MHD K-H instability driven by the shear in the E × B drift velocity is not operative. This instability is driven by the shear in the ion diamagnetic drift velocity, which is a nonideal MHD drift in a high-β plasma and is a macroscopic effect not visible at the guiding center level. The two-dimensional stability (k · B0 = 0) of a model subsolar magnetopause is investigated by solving the eigenmode equation for a polygonal ion diamagnetic drift velocity profile with the density ratio across the magnetopause as a parameter. Near the subsolar magnetopause the fastest growing wave or vortex propagates duskward with a phase velocity from 8 km/s to 14 km/s, and the normalized growth rate decreases with an increase in the ratio of the magnetosheath density to the magnetospheric density. The wavelength and period of the fastest growing mode increases with the density ratio. For realistic parameters near the subsolar magnetopause the wave period becomes 750 s to 2000 s and the wavelength becomes 11000 km to 16000 km. The present K-H instability (“diamagnetically driven K-H instability”) may cause a plasma transport across the subsolar magnetopause, since the plasma motion is decoupled from that of the magnetic field owing to nonideal MHD. We discuss a possible dawn-dusk asymmetry (caused by the ion diamagnetic drift velocity at the magnetopause) of the K-H instability when the present instability is extended to the dayside magnetopause off the noon meridian, where the tailward E × B drift is no longer negligible. The vortex created by the present instability near the subsolar magnetopause has the same rotational sense as that created by the E × B shear driven K-H instability within the dusk flank boundary but has the opposite rotational sense to that created by the E × B shear driven K-H instability within the dawn flank boundary.