Coupled Thermosphere-Ionosphere Tongue-Like Structure During the Recovery Phase of the Geomagnetic Storm on 12 May 2021

A coupled thermosphere-ionosphere tongue-like structure was observed in the daytime at middle latitudes by ground-based Global Navigation Satellite System (GNSS) and Global-scale Observations of the Limb and Disk (GOLD) observations during the storm recovery phase on 12 May 2021. The storm-induced variations in total electron content (TEC) and the column density ratio of O to N2 (& sum;O/N2) both showed a relatively narrow region of enhancement in latitude, originating from low latitudes and surrounded by depleted regions. The concurrent neutral temperature observations also showed a tongue-like decrease surrounded by temperature enhancements. The Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM) qualitatively reproduced the observed thermosphere-ionosphere structure. The simulations indicate that the formation of the TEC tongue-like structure was dominated by the enhanced Sigma O/N2. Unlike the previously reported tongue of ionization (TOI) driven by E x B drifts, the tongue-like structure occurred at relatively lower geomagnetic latitudes (similar to 30 degrees $\mathit{{}<^>{\circ}}$-similar to 65 degrees $\mathit{{}<^>{\circ}}$) and had a smaller magnitude of enhancement. This is the first time that the concurrent tongue-like structures in thermospheric composition, temperature and electron density are observed, which advances the understanding of thermosphere-ionosphere coupling.