Solar flux variation of the electron temperature morning overshoot in the equatorial F region : EQUATORIAL MORNING OVERSHOOT

Using 8 years of CHAMP satellite observations of the equatorial electron temperature, T-e, we investigate its behavior during the morning overshoot and at ionospheric altitudes below 450 km including its variation with solar activity. The morning T-e has a maximum at the dip equator and decreases gradually with increasing latitude, which is due to the increasing importance of heat conduction as the dip angle becomes larger.

The amplitude of the equatorial morning overshoot T-e decreases with increasing solar flux by about -10 degrees K/solar flux unit depending on season and longitude. Trends of similar magnitude are predicted by the FLIP model. The model calculations confirm that the electron cooling due to enhanced electron-ion collisions increases faster than the heating of thermal electrons through collision with photoelectrons for increasing solar EUV.

Both data and model showed that elevated electron temperatures persist to later local times during low solar activity. Obviously, the decreased background plasma density, together with the slower rise of electron density after sunrise under such conditions are responsible for the longer persistence.

First investigations of longitudinal aspects revealed that the strength of the anticorrelation between morning T-e and solar flux and the seasonal difference of the T-e amplitude varies with longitude. The positive correlation between the morning overshoot and solar flux at 600 km as was shown earlier in Hinotori data is consistent with FLIP predictions and radar observations.

The solar flux variation of the morning T-e reverses sign between 400 and 600 km.