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

The problem of nonlocal lower-hybrid drift instability in the magnetotail-l ike 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 she et of the Harris type with constant cross-held flow speed profile, v = v(0) = const, is stable with respect to perturbations in the lower-hybrid frequ ency range, if the normal field component is finite, B-n > 0. They then dis cussed a sheared flow speed profile, v(z) = v(0)/[1+(z/Delta)(2)], which re presents a much thinner current sheet configuration, and recovered the unst able mode. In this paper, the previous work is extended in two aspects. The effects of the variation of the normal field strength, B-n, on the transit ion from instability to stability in the absence of the shear (Delta = infi nity) is examined systematically. In addition, the eigenmodes with odd symm etry, phi(-z) = -phi (z), are included. These two extensions Show that inde ed, the normal field component B-n has a strong stabilizing influence on th e mode. As the shear parameter Delta is reduced, the threshold value of B-n for stabilization increases in proportion to the value of Delta (-1), whic h 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 mode s shows that the odd symmetry eigenmodes generally possess higher growth ra tes than the even symmetry ones.