NUMERICAL SIMULATIONS AND SIMPLIFIED MODELS OF NONLINEAR ELECTRON INERTIAL ALFVEN WAVES

We describe nonlinear resonance absorption of compressional Alfven wav es in a model magnetosphere. It is shown that the ponderomotive force of excited standing shear Alfven waves can lead to nonlinear saturatio n and spatial structuring of field line resonances and that in low-bet a plasmas ponderomotive saturation may occur before other nonlinear sa turation processes such as the Kelvin-Helmholtz instability. The effec ts of finite electron inertia are also considered, and it is shown tha t spatial structuring of field line resonances may occur with scale si zes compatible with those found in discrete auroral arcs. Results are discussed in the context of three-dimensional numerical solutions to t he resistive magnetohydrodynamic equations and based on a simplified a nalytical model of nonlinear resonance absorption.