Stabilization of lower‐hybrid drift instability in the magnetotail by finite north‐south magnetic field component and destabilization by sheared cross‐field flow

Abstract

The problem of nonlocal lower‐hybrid drift instability in the magnetotail‐like geometry characterized by a finite north‐south (normal) magnetic field component at the center of the neutral sheet is revisited. In a preliminary paper [Lui et al., 1995] it was shown that the one‐dimensional neutral sheet of the Harris type with constant cross‐field flow speed profile, v = v0 = const, is stable with respect to perturbations in the lower‐hybrid frequency range, if the normal field component is finite, Bn > 0. They then discussed a sheared flow speed profile, v(z) = v0/[1 + (z/Δ)2], which represents a much thinner current sheet configuration, and recovered the unstable mode. In this paper, the previous work is extended in two aspects. The effects of the variation of the normal field strength, Bn, on the transition from instability to stability in the absence of the shear (Δ = ∞) is examined systematically. In addition, the eigenmodes with odd symmetry, ϕ(−z) = − ϕ(z), are included. These two extensions show that indeed, the normal field component Bn has a strong stabilizing influence on the mode. As the shear parameter Δ is reduced, the threshold value of Bn for stabilization increases in proportion to the value of Δ−1, which leads to the conclusion that the shear in the cross‐field drift speed has a destabilizing influence on the mode. Comparison of the two symmetry modes shows that the odd symmetry eigenmodes generally possess higher growth rates than the even symmetry ones.