EISCAT Observations of Depleted High‐Latitude F‐Region During an HSS/SIR‐Driven Magnetic Storm

Abstract The effect of storms driven by solar wind high‐speed streams (HSSs) on the high‐latitude ionosphere is inadequately understood. We study the ionospheric F ‐region during a moderate magnetic storm on 14 March 2016 using the EISCAT Tromsø and Svalbard radar latitude scans. AMPERE field‐aligned current (FAC) measurements are also utilized. Long‐duration 5‐day electron density depletions (20%–80%) are the dominant feature outside of precipitation‐dominated midnight and morning sectors. Depletions are found in two major regions. In the afternoon to evening sector (12–21 magnetic local time, MLT) the depleted region is 10–18 magnetic latitude (MLAT) in width, with the largest latitudinal extent 62–80 MLAT in the afternoon. The second region is in the morning to pre‐noon sector (04–10 MLT), where the depletion region occurs at 72–80 MLAT within the auroral oval and extends to the polar cap. Using EISCAT ion temperature and ion velocity data, we show that local ion‐frictional heating is observed roughly in 50% of the depleted regions with ion temperature increase by 200 K or more. For the rest of the depletions, we suggest that the mechanism is composition changes due to ion‐neutral frictional heating transported by neutral winds. Even though depleted F ‐regions may occur within any of the large‐scale FAC regions or outside of them, the downward FAC regions (R2 in the afternoon and evening, R0 in the afternoon, and R1 in the morning) are favored, suggesting that downward currents carried by upward moving ionospheric electrons may provide a small additional effect for depletion.