Diurnal variations on a plasmaspheric flux tube: Light ion flows and F region temperature enhancements

Abstract

Diurnal variations on a plasmaspheric flux tube were modelled using a time‐dependent, multi‐species, one‐stream, interhemispheric model for plasmaspheric flows. In the model the coupled time‐dependent hydrodynamic equations (continuity, momentum, and energy) of a two ion (H + and O + ) quasineutral, currentless plasma are solved for a closed geomagnetic field line; no diffusive equilibrium assumptions are made. For this work an L=2 field line with conjugate points displaced in latitude and longitude was used in order to model the tilt of the geomagnetic field. The simulation was done for June solstice conditions during solar minimum. In general the results reproduce those found by Richards and Torr [1986]. However, a striking new result is the presence of large downward H + velocities (∼2.2 km/s) at about 320 km altitude in the winter (southern) hemisphere, in early morning when the summer hemisphere is already sunlit but the winter hemisphere is still in darkness. These are accompanied by large H + temperature enhancements, reaching 5600 K, at about 280 km altitude. The large velocities are a resilt of an increase in the H + pressure gradient coupled with a decrease in the friction encountered by the H + ions. These flows are stopped at lower altitudes where the neutral densities are higher; the bulk kinetic energy is converted to thermal energy, resulting in the temperature enhancements.