A comparative study of plasma expansion events in the polar wind

Abstract

The temporal characteristics of the polar wind were studied with both a full set of hydrodynamic equations and a simplified set of collisionless equations. The hydrodynamic set contained continuity, momentum and energy equations for H+, O+ and electrons, including collisional conductivities and variable, but isotropic, ion and electron temperatures. The collisionless model included the same species and was able to take account of constant anisotropic ion temperatures. The two polar wind models were solved with different numerical techniques and the results were compared for similar expansion scenarios. In particular, the temporal evolution of the polar wind was modelled for initial, extended density depletions of factors of 3, 10 and 100 starting at several altitudes. Also, the two models were used to study the temporal response of the polar wind to varying electron temperature conditions. From this comparative study, we found the following: (1) qualitatively, the two models predict the same temporal characteristics for the polar wind densities, flow velocities, and escape fluxes; (2) the propagation velocities of the disturbances vary markedly with the level of the depletion, with greater depletions producing greater disturbance velocities; (3) for small depletions the H+ disturbance does not steepen into a shock; (4) the rapidly moving H+ disturbances do not significantly affect the O+ distribution, but the slower moving O+ shocks generate secondary H+ disturbances that propagate with the heavy ion shock waves; (5) for a 100fold depletion, the rapid acceleration of the minor H+ ions through the O+ shock acts to enhance the H+ density at higher altitudes to such an extent that H+ becomes the dominant ion over a limited altitude range ahead of the O+ shock; (6) the adiabatic cooling associated with the upward propagation of the disturbances leads to a significant reduction in the O+ temperature, and a less dramatic reduction in the H+ temperature; and (7) the polar wind expansion characteristics are strongly modulated when the electron temperature variations are of the order of five minutes. The density distribution becomes highly structured, and regions of upward and downward ion flows follow the temporal variations of Te. Such a situation can occur when the interplanetary magnetic field is northward and the magnetospheric convection pattern is turbulent.