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 1989-1994 are analysed to investigate the local time and altitude (1000-8000 km) variations of electron temperature at magnetic latitudes 0-45°N. The observed electron temperature, Te, 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 difference (∼ 6500 K/2600 K) occurs at the highest altitude at equatorial latitudes and the smallest difference (∼ 3300 K/2300 K) at the lowest altitude at mid latitudes. During daytime, Te increases rapidly with altitude in the lower plasmasphere (altitude < 2500 km) and slowly in the upper plasmasphere with mean gradients of 1.33 K km-1 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 Te increases slowly with a mean gradient equal to the daytime value.
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