Behavior of ionized plasma in the high latitude topside ionosphere: The polar wind

Abstract

We have developed a numerical model to study the steady-state behavior of a fully ionized plasma (H+, O+ and the electrons) encompassing the geomagnetic field lines extending from 1500 km to 10 Re. The theoretical formulation is based on the 16 moment system of transport equations. The 16 moment equations, which allow transverse and parallel thermal energy to be transported separately and are based on the assumption that the distribution function is nearly bi-Maxwellian, are expected to simulate anisotropy for a collisionless plasma better than the 13 moment equations, used in some previous studies of the polar wind, which allow only a single heat flow per species and are based on simple Maxwellian distributions. We find that in the cases studied the results of 13 and 16 moment simulations are surprisingly similar, even though the temperature anisotropy is quite large. Our studies of the polar wind show that the electron temperature anisotropy develops around 2500 km with T⊥ > T|. Below 2500 km the collisions keep the electron temperature isotropic. The H+ ion temperature exhibits adiabatic cooling with anisotropy T| > T⊥ The results of our polar wind simulations are in good agreement with recent experimental observations. These, to our knowledge, are the first successful steady-state solutions of the 16 moment system of transport equations for the polar wind. © 1987.