NORMAL-MODES OF AN EXPANDING PLASMA SHELL

We present a cold plasma, two-fluid electromagnetic theory of the norm al modes of a plasma shell expanding, at sub-Alfvenic velocities, into a magnetic field. A two-fluid approach is used in order that the spec ific effects of plasma and magnetic field geometry can be considered. Three main results are obtained. First, a general normal mode equation for lower-hybrid frequency range oscillations is derived which is mor e accurate in its treatment of the plasma and magnetic field geometry than previous published normal mode equations. The higher accuracy acc rues because no a priori assumptions are made about the ratio of pertu rbation and equilibrium scale lengths. The second set of results is th e establishment of specific criteria for the existence of localized so lutions of the lower-hybrid drift normal mode equation and the develop ment of analytical formulae which describe the dispersive properties o f this instability. These are useful because results concerning lower- hybrid drift instability growth rates and maximal growing wave numbers are usually given in the context of the local approximation without s pecifying where the normal modes are localized within the highly inhom ogeneous plasma profile. The third result is a cautionary one. Two equ ilibrium models are constructed with identical magnetic field profiles , similar density profiles, but distinctive electron cross-field veloc ity profiles. The localization criteria and analytical formulae for lo wer-hybrid drift dispersive properties are found to be quite accurate for one equilibrium model but to be inappropriate for the other.