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 degrees 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 (>10(6) 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 degrees) 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 F(2) peak N(e).
