A comparison of the temperature and density structure in high and low speed thermal proton flows

Abstract

The continuity, momentum and energy hydrodynamic equations for an H +-O + topside ionosphere have been solved self-consistently for steady state conditions similar to those found outside the plasmasphere. Results are given for undisturbed and trough conditions with a range of H + outflow velocities yielding subsonic and supersonic flow. In the formulation of the equations, account was taken of the velocity dependence of ion-neutral, ion-ion and ion-electron collision frequencies. In addition, parallel stress and the nonlinear acceleration term were retained in the H + momentum equation. Results computed from this model show that, as a result of Joule (frictional) heating, the H + temperature rises with increasing outflow velocity in the subsonic flow regime, reaching a maximum value of about 4000 K. For supersonic flow other terms in the H + momentum equation become important and alter the H + velocity profile such that convection becomes a heat sink in the 1000–1500 km altitude range. This, together with the reduced Joule heating resulting from the high-speed velocity dependence of the H + collision frequencies, results in a decrease in the H + temperature as the outflow velocity increases. However, for all outward flows the H + temperature remains substantially greater than the O + temperature. With identical upper boundary velocities, the H + flow velocity is higher at low altitudes for trough conditions compared with non-trough conditions, but the H + temperature in the trough is lower. The form of the H + density profiles for supersonic flow does not in general differ greatly from those obtained with wholly subsonic flow conditions.