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 N 2 (∑O/N 2 ) 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 ΣO/N 2 . Unlike the previously reported tongue of ionization (TOI) driven by E × B drifts, the tongue‐like structure occurred at relatively lower geomagnetic latitudes (∼30–∼65) 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.