Plasmasphere electron temperature studies using satellite observations and a theoretical model

Electron temperature variations in the Earth's plasmasphere are studied using the Exos D satellite observations and the Sheffield University plasmasphere-ionosphere model. The observations made during the years 1989-1994 are analyzed to investigate the local time and altitude (1000-8000 km) variations of the electron temperature at magnetic latitudes 0 degrees-45 degrees N. The observed electron temperature T-e is almost constant during both day and night and is found to have large day-to-night differences that vary with altitude and latitude; the largest day-to-night ratio in T-e (approximate to 6500 K/2600 K) occurs at the highest altitude at equatorial latitudes and the smallest ratio (approximate to 3300 K/2300 K) at the lowest altitude at midlatitudes. During daytime, T-e increases rapidly with altitude in the lower plasmasphere (altitude <2500 km) and slowly in the upper plasmasphere with mean gradients of 1.33 and 0.22 K km(-1), respectively. At night, on the other hand, the lower plasmasphere is in thermal equilibrium, and in the upper plasmasphere T-e increases slowly with a mean gradient equal to the daytime value. The electron temperature shows maximum latitude variation at medium plasmaspheric altitudes (around 4000 km) and smaller variations at lower and higher altitudes, particularly during daytime. At the altitude of maximum latitude variation, T-e increases by about 1200 K between the equator and 40 degrees N during both day and night. The model reproduces the observations reasonably well and is used to explain the occurrence of a prominent morning peak in T-e. The morning peak becomes prominent in the lower plasmasphere at low latitudes because of the vertical E x B drift in the equatorial F region, which increases T-e during morning hours and decreases T-e during daytime hours. The observations and model results also show the occurrence of an afternoon peak in T-e, which becomes prominent with increasing altitude and latitude; the peak could be caused by the daytime poleward neutral wind.