Bounce-Averaged Stability Analysis of High Beta Flux Tubes

A novel numerical procedure has been developed for analyzing the stability of a particular flux tube to ballooning-interchange modes in plasmas for which the beta value β is very high O(1 ~ 10). The stability of a stressed dipole configuration in response to arbitrary fully electromagnetic perturbations is studied, including the effects of finite larmor radius, parallel electric fields, arbitrary complex frequency, and trapped/untrapped particle populations. This procedure has been applied to the problem of determining the trigger mechanism for magnetospheric substorms in the stressed geomagnetic tail where realistic equilibrium fields are used to compute the bounce averaged curvature and grad-B drifts. Previously, MHD-like modes were found to be unstable between two critical β 's, (β ₁, β₂). Local analysis concluded that instabilities due to kinetic resonances might persist above β ₂. In this work the local approximation is dropped yielding a large matrix eigenvalue problem for which a parallel machine is well suited. Results are compared to different fluid models that include Hall and finite larmor radius effects. The energy released by such unstable modes is calculated and compared with measurements of auroral brightening. This work was supported by the National Science Foundation Grant ATM-9907637 and the U.S. Dept. of Energy Contract No. DE-FG03-96ER-54346.