Investigation of different ΣO/N2 variations observed by GOLD during a minor geomagnetic storm from 2 to 4 August 2021

During a minor geomagnetic storm occurring from Aug 2 (day-of-year (DOY) 214) to Aug 4 (DOY 216), 2021, the National Aeronautics and Space Administration Global-scale Observations of the Limb and Disk (GOLD) mission observed different column density ratio of O to N-2 (Sigma O/N-2) variations in storm main and recovery phases. The percentage difference of Sigma O/N-2 between DOY 215 (disturbed day, main phase) and DOY 213 (quiet reference day) exhibits a depletion on the east side of the GOLD field-of-view (FOV). However, that of Sigma O/N-2 between DOY 216 (recovery phase) and 213 shows depletions on the west side of GOLD FOV. The National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model qualitatively reproduced the observations. Analysis of the model output illustrates that the Sigma O/N-2 patterns in the two days are both formed due to the classical thermospheric composition theory and formed on DOY 214 and 215, respectively. Further investigation found that the Sigma O/N-2 depletion on DOY 214 and 215 both initially formed near 120-180 degrees E, but the one on DOY 215 then quickly moved westward into the GOLD FOV, from local post-midnight to pre-midnight, near 19 UT. Then it moves equatorward and slowly westward. This results in the observed depletion structure on the west side of GOLD FOV. Model simulations show that the quick westward movement near 19 UT is due to the dominant positive Interplanetary Magnetic Field east-west component (B-y) conditions.