Plasmasphere electron temperature profiles and the effects of photoelectron trapping and an equatorial high-altitude heat source

The electron temperature T-e in the Earth's plasmasphere up to 10,000 km altitude, has been measured routinely by a thermal electron energy distribution instrument on board the EXOS D satellite. The measurements made during the years 1989-1995 have been analyzed to obtain altitude (1000-8000 km) profiles of T-e for magnetic latitudes 0 degrees-40 degrees N at different times of the day. The profiles are compared with those computed by the Sheffield University plasmasphere-ionosphere model, modified to include nonlocal heating due to trapped photoelectrons and an equatorial high-altitude heat source. The photoelectrons are trapped at altitudes above 600 km, and the high-altitude heat source is applied as energy input along the magnetic field lines with apex altitude greater than 600 km between +/-10 degrees magnetic latitude. The results for 8000 km altitude show that the modeled values of T-e computed without photoelectron trapping and the high altitude heat source are much less than the mean measured values, 3700 K compared with 6500 K. Depending upon altitude and latitude, a photoelectron trapping of up to 100% is required to raise the modeled electron temperatures to the mean measured values. However, photoelectron trapping alone cannot account for the observed latitude variation of T-e, which depends on altitude. Model calculations carried out with 50% photoelectron trapping and an altitude and local time-dependent high-altitude heat source reproduce the measurements, including the latitude variation; the heat source required for nighttime is about one fourth of that required for daytime. An equatorial high-altitude heat source appears to be the only mechanism that can account for the measured values of T-e; values in excess of 12,000 K have been measured. There are some quantitative differences between the measured and modeled temperatures at night in the lower plasmasphere (<2500 km), which could be caused, in whole or in part, by the inaccuracies of the nighttime measurements due to the low plasma densities.