Electrostatic probe measurements of electron temperature from the Isis 1 satellite for the equinox season of high solar activity years 1969 and 1970 are analyzed for the 1000-km altitude. These results are then compared with the Explorer 22 satellite measurements of electron temperature for the years 1965--1967. Large solar activity changes are found- to occur in the latitudinal profiles of electron temperature during both day and night. At low and mid-latitudes, the electron temperature increases from about 2000 to about 3500¿K for the daytime and from about 800 to about 1700¿K during the nighttime between low and high solar activity conditions. At high latitudes there is an increase in electron temperature by about a factor of 2 from low to high solar activity period. These solar activity changes in electron temperature are interpreted to be mainly due to the heat flux conducted down from the protonosphere, which increases with solar activity.