Diagnostic Analysis of the Physical Processes Underlying the Long‐Duration Σ $\boldsymbol{\Sigma }$ O/N2 Depletion During the Recovery Phase of the 8 June 2019 Geomagnetic Storm

A thermospheric O and N2 column density ratio (∑O/N2) depletion with long‐duration (>16 hr) was observed by the Global‐scale Observations of the Limb and Disk at the Atlantic longitudes (75 ° ${}^{\circ}$ W–20 ° ${}^{\circ}$ W) and middle latitudes (20N–50 ° ${}^{\circ}$ N) during the recovery phase of the 8 June 2019 geomagnetic storm. The National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) simulations reproduced the ∑O/N2 depletion patterns with a similar magnitude, and indicated that the composition recovery at middle latitudes began several hours after the beginning of the recovery phase of the geomagnetic storm. The TIEGCM simulations enable quantitative analysis of the physical mechanisms driving the middle‐latitude composition changes during the storm recovery phase. This analysis indicates that vertical advection and molecular diffusion dominated the initial recovery of composition perturbations at middle latitudes. Horizontal advection was also a main driver in the initial recovery of composition, but its contribution decreased rapidly. In the late recovery phase, the composition recovery was mainly determined by horizontal advection. In comparison, vertical advection and molecular diffusion played a much less important role.