Correlation between electron density and temperature in the topside ionosphere

The correlation between the electron temperature (T_e) and electron density (N_e) at 600 km height at magnetic dip latitudes (MLat) less than about ±40° measured by the Hinotori satellite from February 1981 to June 1982 is presented. The results show the well-known negative correlation between daytime N_e and T_e when N _e is low. However, when the daytime N_e is significantly high (>10⁶ cm^-3), the correlation turns positive irrespective of latitude, longitude, season, solar flux levels, and magnetic activity levels. The positive correlation is most clear during 1100-1500 local time and around the magnetic dip equator (MLat < 10°) where high values of N_e with high T_e are most abundant. T_e also increases with increasing MLat in the same N_e range. Since the cooling through Coulomb collisions increases with the increase of N_e, the results suggest that an additional heat source(s) is involved for the positive correlation between N_e and T_e. The additional heat source seems to be related to the integrated N_e along the magnetic field lines from the ground to 600 km altitude in one hemisphere, which is found to decrease with increasing MLat. Although the mechanism for the positive correlation is not well understood, the results imply that the T _e in the topside ionosphere is controlled more by the integrated N_e than by in situ N_e or F₂ peak N _e..