HYDROMAGNETIC EQUILIBRIUM AND INSTABILITIES IN THE CONVECTIVELY DRIVEN NEAR-EARTH PLASMA SHEET

Recent particle simulations have suggested that the convectively drive n near-Earth plasma sheet can develop a structure in which a thin curr ent sheet is embedded within the much thicker plasma sheet and that fi nite-ky modes with the character of kink and interchange modes can be excited in this system. Here the ideal magnetohydrodynamic (MHD) equat ions are used to investigate the equilibrium and linear stability prop erties of such a system. It is shown that the embedded current sheet c onfiguration satisfies the conditions of pressure balance and represen ts an approximate two-dimensional (x,z) MHD equilibrium state. The sta bility analysis, in which the boundary conditions are imposed in terms of MHD characteristic waves, indicates that two types of pressure-dri ven modes are unstable. One mode is associated with the presence of a tailward gradient in the equatorial magnetic field profile. As the wav elength of this mode is made shorter, the mode becomes localized on th e field lines crossing the region of increasing field and has the char acter of an interchange/ballooning mode. A second mode is associated w ith the existence of a magnetic island and is localized within the isl and; it has a structure similar to that of the classical kink mode for a plasma column. While the MHD growth rates continue to increase at s hort wavelengths, at longer wavelengths determined by the ion gyroradi us (k(y) rho(i) less than or similar to 1, where rho(i) is computed in the local equatorial field) they are comparable to those for the domi nant nonlinear modes observed in the kinetic simulations.