A linear analysis of the hybrid Kelvin‐Helmholtz/Rayleigh‐Taylor instability in an electrostatic magnetosphere‐ionosphere coupling system

Abstract

A linear analysis of the hybrid Kelvin‐Helmholtz/Rayleigh‐Taylor (KH/RT) instability in the magnetosphere‐ionosphere coupling system is presented. In the magnetosphere plasma particles can move by the electric (E × B) drift, magnetic drift (gradient B drift plus curvature drift), and inertia drift. Field‐aligned currents are generated primarily from divergence of the magnetic drift flux. They are closed via Pedersen currents in the ionosphere. As a simple model, there is a surface of discontinuity both in the azimuthal flow and the particle energy density, which extends longitudinally as projected to the ionospheric plane. The gradient of the energy density is directed inward or outward so that the plasma is RT unstable or stable, respectively. The present analysis shows that only a velocity shear cannot drive the system KH unstable when the growth rate, γKH, of the KH instability without Pedersen coupling is less than the inertial relaxation rate, ν, and that the presence of an energy density gradient allows a hybrid wave to grow even when γKH < ν. The wave growth is due to the active charge separation driving originally the RT instability. The picture of auroral deformations now changes drastically. The traditional picture is such that if the Pedersen coupling is properly taken into account, KH waves of long wavelengths (≳100 km at the ionospheric height) are found evanescent in the main body of the auroral oval. A new one is such that wavy structures therein can develop, only in the KH/RT hybrid mode, at the places where the particle energy density has an inward gradient. Similarly, the stability of the inner boundary of the low‐latitude boundary layer is controlled by the RT stability conditions more influentially than the KH conditions.