Linear and nonlinear dispersive effects on magnetospheric field line resonances

Shear Alfvén wave dispersion produced by electron inertia, ion gyro-kinetic and electron thermal pressure is modeled using two-fluid MHD and kinetic theory. In a dipolar magnetosphere, dispersion and non-linearity determine the spatial structure, temporal evolution and amplitude of parallel electric fields and large amplitude density fluctuations near to the polar ionosphere. Deep auroral density cavities are found to have a strong influence on auroral electric field generation. Many features of satellite and ground based observations of discrete arcs are predicted using two-fluid MHD, but large parallel electric fields (mV/m) and keV electron precipitation cannot easily be explained. To explain the observed electric fields it is necessary to evaluate the non-local kinetic electron response to standing shear Alfvén waves on dipolar magnetic field lines. It is shown that electron trapping leads to a significant reduction of the collisionless electron conductivity and a large enhancement of parallel electric fields in the 1 – 4 mHz frequency range of observed field line resonances.