A brief review of the equatorial thermosphere anomaly

The equatorial thermosphere anomaly (ETA), a similar phenomenon to the equatorial ionization anomaly (EIA), is a feature of the upper thermosphere with two crests at ±20~30° in magnetic latitude and a trough at the magnetic equator. Albeit the ETA was first reported in the N2 observations from the OGO6 satellite back in the 1970s, the causes of the ETA formation and its variability are not well understood. Recent studies have revealed that neither heat transport due to zonal winds nor chemical heating can explain the formation of the ETA crests. Instead, it was found that plasma-neutral collisional heating and the field-aligned ion drag are the major contributors in producing the ETA crest and trough respectively. A brief review was given on some significant achievements gained to the mechanisms of the ETA formation and questions remaining about the physical processes for the ETA variability, which should be explored in the near future.