Equatorial temperature anomaly during solar minimum

We show evidence for the occurrence of the equatorial temperature anomaly ( ETA) during solar minimum by analyzing the temperature and total ion densit y data from the Neutral Atmosphere Temperature Experiment (NATE) and the Cy lindrical Electrostatic Probe (CEP), respectively, on board the Atmospheric Explorer-E satellite. The chosen data refer to a height of similar to 254 km in the African and Asian longitude sector (340, I degreesE-200 degreesE) during a summer season in the Southern Hemisphere. As during the solar max imum period, the spatial characteristics of the ETA are similar to those of the equatorial ionization anomaly (EIA). A minimum in the gas temperature is collocated with the minimum in the ion density at the dip equator, and a temperature maximum on the south side of the equator is collocated with th e density maximum of the EIA. The daytime behavior of ETA formation is abou t the same as that of EIA as both of them are clearly present at around 130 0 and 1400 local solar time (LST) only. At 1400 LST the difference between the temperatures at the crest and the trough (ETA strength) reaches a maxim um value of about 100 degreesK which is similar to 14% of the temperature a t the trough. Like the EIA, the ETA also suddenly disappears after 1400 LST , Thus the EIA appears to be a prerequisite for the ETA formation. During t he premidnight time (2200 LST), however, while the EIA is nonexistent, the temperature distribution forms a pattern opposite to that at 1400 LST in th e daytime. It shows a maximum around the dip equator and a broad minimum at the daytime crest region where the postsunset cooling also is faster and o ccurs earlier than at the dip equator. This nighttime maximum appears to be related to the signature of the midnight temperature maximum (MTM). Mass S pectrometer Incoherent Scatter (MSIS) model temperatures, in general, are h igher than the observed average temperatures for the summer season and in p articular for the region around the dip equator around noon hours.