Simulations of the equatorial thermosphere anomaly: Physical mechanisms for crest formation

Abstract

Several mechanisms including heat transport due to zonal winds, chemical heating and field‐aligned ion drag have been proposed to explain the formation of the Equatorial Thermosphere Anomaly (ETA), but the cause of the ETA crests in thermosphere temperature is still a mystery. Our companion study (Lei et al., 2012) has revealed that the field‐aligned ion drag mainly contributes to the ETA trough, but has little effect on the ETA crests. In this study, the mechanisms of heat transport associated with zonal winds and chemical heating due to recombination are examined to assess their contributions to the production of the ETA crests on the basis of National Center for Atmospheric Research Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (NCAR‐TIEGCM) simulations. Our sensitivity simulations demonstrate that neither heat transport due to zonal winds nor chemical heating is able to explain the formation of the ETA crests. Instead, we found that the formation of the ETA crests is attributed to plasma‐neutral heating which has two peaks in the topside ionosphere aside the magnetic equator. These two peaks, which are largely controlled by the magnetic field, are the results of energy transfer from thermal electrons and ions to the neutrals through collisions due to their temperature differences, albeit the ultimate source of this heating is solar radiation which produces photoelectrons that mainly depend on solar zenith angle. The TIEGCM simulations show that the crests of the ETA always locate poleward by 10°–15° with respect to those of the Equatorial Ionosphere Anomaly (EIA), although the trough location of the ETA resembles that of the EIA. The location of the ETA crests is associated with the two‐hump structure in plasma‐neutral collision heating which is small inside the EIA region and larger at the poleward edge of this region.