Unusual Enhancement in He(2 3 S) Airglow Brightness Associated With Interplanetary Magnetic Field Northward Turnings During the 2025 New Year's Day Storm

Abstract The upper thermosphere is important in the context of space weather, but continuous monitoring of the thermosphere at high latitudes remains limited despite its strong variations associated with geomagnetic activity. Orthohelium, He(2 3 S), is found from the upper thermosphere to lower exosphere, and is a promising tracer of atmospheric dynamics. Here we report on unusual enhancements in He(2 3 S) airglow during a severe geomagnetic storm on New Year's Day in 2025, associated with impulsive changes in the solar wind. Multi‐spectral optical observations show that the enhancement at the cusp coincided with electron and proton aurora and an increased density at an altitude of 850 km. The morphology of the He(2 3 S) airglow closely matched that of electron aurora. The observed enhancement was attributed to electron precipitation and recombination of upwelling , while charge exchange of precipitating and ‐particles had a minor role in causing the airglow.