Diurnal, seasonal, latitudinal and solar cycle variation of electron temperature in the topside F-region of the Indian zone ionosphere

Electron temperature T, observed by the SROSS C2 satellite at equatorial and low latitudes during the low to high solar activity period of 1995-2001 at the height of similar to 500 km is investigated in terms of local time, season, latitude, solar sunspot number R, and F10.7crn solar flux. The satellite covered the latitude belt of 31 degrees S-34 degrees N and the longitude range of 40 degrees -100 degrees E. The average nighttime (20:00-04:00 LT) T, varies between 750-1200K and then rises sharply in the sunrise period (04:00-06:00 LT) to the morning high from 07:00 to 10:00 LT and attains a daytime (10:00- 14:00 LT) average of 1100-2300 K. The morning enhancement is more pronounced in the equinoxes. A secondary maximum in T-e is also observed around 16:00-18:00 LT in the June solstice and in the equinoxes. Daytime electron temperature was found to be higher in autumn compared to that in spring in all latitudes. Between the solstices, the amplitude of the morning enhancement is higher in winter compared to that in summer. Both day and nighttime T-e observed by the SROSS C2 satellite bears a positive correlation with solar activity when averaged on a shorter time scale, i.e. over the period of a month. But on a longer time scale, i.e. averaged over a year, the daytime electron temperature gradually decreases from 1995 till it reaches the minimum value in 1997, after which T-e again continues to rise till 2001. The variations are distinctly seen in summer and in the equinoxes. The sunspot activity during solar cycle 23 was minimum in 1996 and maximum in 2000. Annual average electron temperature, therefore, appears to follow the variation of solar activity with a time lag of about one year, both at the bottom and top of solar cycle 23, indicating an inherent inertia of the ionosphere thermosphere regime to variations in solar flux.