Early satellite measurements in the F region found a peak in electron temperature, Te, at night on L shells that thread the expected location of the equatorial plasmapause. This Te enhancement has been attributed to heating by magnetospheric sources. The Te peak typically occurs near 60¿ invariant latitude, moves equatorward during magnetically active periods, and recovers during quieter times in a fashion similar to observed plasmapause behavior. Recent Dynamics Explorer 1 and 2 measurements near 2100 LT have shown that the Te peak in that sector occurs on L shells that are either within the gradient of the plasmapause or just equatorward of that location. Dayside Te signatures in the F region near 60¿ invariant latitude also have been reported, but these are observed primarily above 600 km, and their relationship to dayside plasmaspheric features, if any, has been unknown. In this paper we employ a more extensive set of DE-1 and -2 measurements to examine this relationship at all local times. Statistically, the diurnal variation of the F region Te signature and the plasmapause at altitudes of the order of 104 km are essentially identical between 2200 and 1300 LT, but they diverge in the afternoon sector. The quiet time Te signature remains in the vicinity of 60¿ at all local times, while the plasmapause itself bulges to about 70¿ at 1500 LT. Simultaneous DE-1 observations, however, show that the plasmasphere in the bulge region often exhibits an internal feature in the vicinity of 60¿, which takes the form of a sharp gradient in H+. In each case examined in detail this gradient marked the L coordinate of an F region Te ledge which has higher temperatures on the equatorward side. Te signatures are not usually seen at the plasmapause itself which is at higher latitudes in this local time sector.

The light ion gradient seems to represent a sharp boundary between an old plasmasphere and a new plasmasphere that has been created too recently for refilling to have washed out the sharp boundary between them. This boundary has sometimes been called the inner plasmapause. The latitudinal movement of the Te signature with magnetic activity is similar to all local times but is more sensitive to kp above L=4 and less sensitive below L=4. The Te signatures are seldom observed below L=2.5. These characteristics of the Te signature are qualitatively consistent with plasmasphere depletion and refilling time constants. The Te enhancements at all local times appear to have their quiet time source in the photoelectron energy that is stored in the thermal plasmaspheric electrons. During magnetic substorms an additional magnetospheric heat source enhances the Te signatures, perhaps a result of penetration of the ring current into the plasmasphere. ¿ American Geophysical Union 1988