ETA Variability and Its Coupling with the EIA During Sudden Stratospheric Warmings

The two prominent dayside features of the low-latitude ionosphere and thermosphere (IT) are the Equatorial Thermosphere Anomaly (ETA) and the Equatorial Ionization Anomaly (EIA). EIA is characterized by twin plasma density crests at ~ ±15° and a trough at the magnetic equator in the F-region (Appleton, 1946). ETA manifests in the upper thermosphere, as a pronounced mass density trough at the dip equator and two crests located roughly at ±20-30° magnetic latitude (Mayr et al., 1974). Various formation mechanisms have been postulated previously, including field-aligned ion drag for the ETA trough, and neutral-plasma collisional heating for the ETA crests (Lei et al, 2012 a and b). However, the full spectrum of variability and formation mechanisms remains elusive due to limited observations (Wang et al, 2021).

Among the most dramatic lower atmosphere forcings influencing the IT are sudden stratospheric warmings (SSWs). This study investigates ETA variability and its coupling with EIA during SSWs. SSWs are known to alter the neutral wind structures that extend from the troposphere upward through the IT (Pedatella et al., 2023 and the references therein), but their impact on ETA remains largely unexplored. A case study of the January 2019 SSW provided the first observational evidence of complete ETA disappearance, with EIA vanishing ~3 days after SSW onset and ETA ~3 days later. This sequential disappearance suggested a dual-mechanism framework: meridional ion drag as the primary ETA driver and SSW-amplified tides as a secondary sustaining mechanism capable of decoupling ETA from EIA. These findings establish a new form of ETA variability and a novel tracer of whole-atmosphere coupling. Motivated by this, we implement a super epoch analysis across multiple Northern and Southern Hemisphere SSW events using GRACE and GRACE-FO observations, spanning varying solar flux conditions under geomagnetically quiet periods. This study aims to fill critical gaps in our understanding of low-latitude dynamics by systematically examining how hemisphere, solar flux, and SSW strength collectively govern low-latitude IT coupling.