Seasonal variation of the longitudinal structure of the equatorial ionosphere: Does it reflect tidal influences from below?

We have examined the longitudinal structure of the equatorial ionosphere at 400-km altitude in the noon and postsunset local time sectors in different seasons using 6 years of F-region plasma density observations from the CHAMP satellite. A four-peak wave structure is observed in both local time sectors. In the noon sector at a fixed solar flux level, this structure is observed to be most prominent around September equinox and weakest around December solstice. This seasonal dependence agrees well with that of the nonmigrating diurnal tides DE3, hence supporting a close coupling between the ionosphere and the mesosphere-lower thermosphere possibly via the DE3 tidal modulation of the E-layer dynamo. In the postsunset sector, however, such agreement cannot be claimed. In this sector, although the four-peak structure can be observed in all seasons at moderate and high solar flux levels, its seasonal dependence does not follow that of the DE3 tides. This structure becomes indiscernible near solstices at low solar flux levels. Furthermore, the postsunset four-peak wave structure exhibits larger amplitude than that during daytime, hence indicating that it is more likely an amplified feature rather than a remnant of the daytime structure. The prereversal enhancement is speculated to be a possible candidate to cause this amplification.