Reduction of High Latitude Ionospheric Electron Density by the Impact of Negative Solar Wind Pressure Pulse During the Geomagnetic Storm of 23 March 2023

The G3‐G4 class geomagnetic storm on 23 March 2023, triggered by an unexpected coronal mass ejection (CME), caused a sudden solar wind dynamic pressure drop (SWDPD) event with a substantial drop of more than 15 nPa. This large pressure drop led to global depletion of the ground magnetic field, changes in convection patterns, and a decrease in the total field‐aligned current. Ionospheric electron density variations during the SWDPD were studied using Global Navigation Satellite System (GNSS)‐Total Electron Content (TEC) data in combination with European Incoherent Scatter Scientific Association (EISCAT) radar observations. For the first time, we present the observation of a significant and persistent reduction in TEC at high latitudes across all local time sectors following the SWDPD. EISCAT radar observations at Tromsø (66.7° MLAT) confirmed this reduction, showing a sudden drop in daytime plasma density immediately after the negative pressure pulse. We suggest that electrodynamic modifications from the sudden magnetospheric expansion were the primary driver of TEC depletion in the high latitude ionosphere, although storm‐induced thermospheric changes, such as O/N 2 depletion, also played a role. These findings are crucial for understanding the high latitude ionosphere's response to negative solar wind pressure pulses and highlight the importance of magnetosphere‐ionosphere (M‐I) coupling in ionospheric density variations.