DYNAMIC POLAR WIND AND ITS RESPONSE TO KINETIC ION HEATING

The dynamic polar wind and its response to kinetic ion heating are examined in this paper. We used a time-dependent hydrodynamic polar wind code to study the dynamic flow characteristics of an H+ and O+ polar wind from 200 km to approximately 6 - 7 R(E). We found that the polar wind ion relaxation times are comparable to the time required for the ions to propagate at the ion speed of sound over the altitudinal range considered. Although there may be large transient O+ fluxes lasting approximately 15 min during the early stage of expansion of the unheated polar wind, these fluxes cannot be sustained because of the large O+ gravitational potential. We subsequently studied how polar wind ion heating affects the ion outflow. Our previous work showed that O+ and H+ polar wind ions can become heated due to ion beam instabilities in the polar cap region. We incorporate these kinetic effects into our hydrodynamic model by specifying ion temperature profiles in the ion momentum equations. Two limiting cases were considered, (1) preferential O+ and (2) preferential H+ ion heating. We found that ion heating does not have much effect on the escape of H+ ions because of the limiting nature of the H+ escape fluxes. However, O+ heating can substantially increase the upward O+ pressure gradient in the heating region. This leads to increased O+ ion escape from the polar ionosphere. Thus O+ heating is a viable explanation for the unexpectedly large O+ fluxes which are frequently observed in the polar magnetosphere.