Linear and nonlinear dispersive effects on magnetospheric field line resonances

Shear Alfven wave dispersion produced by electron inertia, ion gyro-kinetic and electron thermal pressure is modeled using two-fluid MI-ID and kinetic theory. In a dipolar magnetosphere, dispersion and non-linearity determine the spatial structure, temporal evolution and amplitude of parallel electr ic fields and large amplitude density fluctuations near to the polar ionosp here. Deep auroral density cavities are found to have a strong influence on auroral electric field generation. Many features of satellite and ground b ased observations of discrete arcs are predicted using two-fluid MHD, but l arge parallel electric fields (mV/m) and keV electron precipitation cannot easily be explained. To explain the observed electric fields it is necessar y to evaluate the non-local kinetic electron response to standing shear Alf ven waves on dipolar magnetic field lines. It is shown that electron trappi ng leads to a significant reduction of the collisionless electron conductiv ity and a large enhancement of parallel electric fields in the 1 - 4 mHz fr equency range of observed field line resonances. (C) 2001 Elsevier Science Ltd. All rights reserved.