Unusual Dawnside Storm‐Enhanced Density and Coincident Neutral Tongue Structure During the 16 April 2025 Geomagnetic Storm

Abstract This study investigates midlatitude ionospheric and thermospheric variations during the 16 April 2025 geomagnetic storm through multi‐instrument analysis and numerical simulation. The analysis uses a comprehensive data set comprising ground‐based Global Navigation Satellite System Total Electron Content (GNSS TEC) and ionosonde measurements, along with space‐based observations from the Swarm, Defense Meteorological Satellite Program (DMSP), Global‐scale Observations of the Limb and Disk (GOLD), and TIMED Global Ultraviolet Imager (GUVI), as well as the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM). We found distinct midlatitude density gradient structures in both the ionosphere and thermosphere during the storm's main phase as follows: (a) An unusual storm‐enhanced density (SED) was observed originating in the dawn sector, which persisted over North American longitudes for 8–9 hr. In contrast, the local afternoon and dusk sectors over Europe showed no SED signature but instead exhibited a negative ionospheric storm phase. (b) Coincident with the morning SED, we observed a distinct tongue‐like structure in neutral composition with O/ enhancement of 20%–40% in the North American sector, which was bounded by areas of O/ depletion. This formed a coherent thermosphere‐ionosphere structure with concurrent enhancement and depletion. (c) The dawnside SED can be largely attributed to two synergistic mechanisms: a chemically driven increase in plasma density from an Oxygen‐rich neutral tongue, coupled with an electrodynamically driven zonal electric field increased by the sunrise enhancement of the penetration electric field. These results underscore the critical role of neutral composition and electrodynamic forcing in the formation of storm‐time midlatitude ionospheric gradient structures, offering new insights into the dynamics of the coupled ionosphere‐thermosphere system.