The Effects of IMF B_y on the Middle Thermosphere During a Geomagnetically “Quiet” Period at Solar Minimum

Numerical simulations using the National Center for Atmospheric Research (NCAR) thermosphere-ionosphere-electrodynamics general circulation model (TIE-GCM) are performed to elucidate the effects of the interplanetary magnetic field (IMF) on the middle thermosphere composition during a “geomagnetically quiet” period from the day of year (DOY) 110–111 in 2019 (when the Auroral electrojet (AE) index never exceeded 300 nT and the Kp never exceeded 2). In particular, this paper aims to examine how the Global-scale Observations of the Limb and Disk (GOLD) mission observed daytime thermospheric O and N₂ column density ratio (∑O/N₂) depletion at mid-latitudes originated under such a “geomagnetically quiet” condition. A comparison of electric potential, Joule heating rate per unit mass, ion velocity, neutral temperature and winds in the middle thermosphere (∼160 km) between real IMF and without the IMF east-west component (B_y) indicates that a B_y dominant condition can enhance their strengths under this “geomagnetically quiet” condition. Consequently, ∑O/N₂ depletion with a stronger magnitude (30% compared with ∼8% without B_y) and larger disturbed area was introduced in the post-midnight sector at high-latitudes due to strong and localized upwelling associated with the enhanced Joule heating rate per unit mass. The ∑O/N₂ depletion was transported equatorward and corotated from local post-midnight to early morning, and was observed by GOLD at middle latitudes during daytime.