A study of the thermal structure of the low-latitude (30 ¿N to 30 ¿S) ionosphere under equinoctial conditions at low, medium, and high solar activity has been carried out using the Sheffield University plasmasphere-ionosphere model (SUPIM) and Hinotori satellite observations. The study reveals the existence of an anomaly in the plasma (electron and ion) temperature in the topside ionosphere during the evening-midnight period. The anomaly, called the equatorial plasma temperature anomaly (EPTA), is characterized by a trough around the magnetic equator with crests on either side. The trough develops before the crests. The model results show that the anomaly occurs between 1900 and 0100 LT at altitudes between 450 and 1250 km; the strongest anomaly occurs around 2130 LT at 950 km altitude during high solar activity. The temperature trough of the anomaly arises from the adiabatic expansion of the plasma and an increase in plasma density caused by the prereversal strengthening of the upward vertical E¿B drift. The temperature crests arise from the combined effect of the reverse plasma fountain and nighttime plasma cooling. The electron temperature measured by the Hinotori satellite near 600 km altitude during medium and high solar activity periods shows the existence of the EPTA with characteristics in close agreement with those obtained by the model. The model also reproduces the occurrence of a daytime temperature bulge in the electron temperature in the bottomside ionosphere; the ion temperature shows no bulge.¿ 1997 American Geophysical Union