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

• Published in: Journal of geophysical research. Space physics Vol. 129; no. 9
• Main Authors: Ellahouny, N. M., Aikio, A. T., Vanhamäki, H., Virtanen, I. I., Cai, L., Marchaudon, A., Blelly, P.‐L., Coster, A., Norberg, J., Maute, A., Oyama, S.‐I.
• Format: Journal Article
• Language: English
• Published: 01.09.2024
• Subjects: EISCAT; electron density depletion; high latitude; high speed streams; ion frictional heating; ionospheric storms
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2024JA032910

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° ^{\circ}$–18° ^{\circ}$ magnetic latitude (MLAT) in width, with the largest latitudinal extent 62° ^{\circ}$–80° ^{\circ}$ 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° ^{\circ}$–80° ^{\circ}$ 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. Plain Language Summary We study the effects of a moderate magnetic storm in March 2016 driven by solar wind high‐speed streams in the high latitude ionosphere. The EISCAT incoherent scatter radar latitudinal scans in Tromsø, Northern Norway, and on Svalbard island are utilized together with other ground‐based and satellite measurements. A strong, long‐duration decrease in electron density at 290 km altitude is observed for the first 5 days of the storm in two distinct local time sectors. In the afternoon to evening local time sector, the depletion covers a broad range of latitudes from the polar cap to the subauroral latitudes, while in the morning to prenoon sector, it covers a smaller range of latitudes, mainly within the auroral oval and extends to the polar cap. We show that ion‐neutral frictional heating due to strong plasma flows plays a major role in generating the depletion in the high‐latitude ionosphere. Additionally, depleted regions favor the downward field‐aligned current regions. Key Points Strong long‐duration F‐peak electron density decrease is seen by EISCAT radars for 5 days during a high‐speed stream driven magnetic storm Depleted F‐region covers a broad latitude range from polar cap to subauroral latitudes in the afternoon‐evening magnetic local time sector Ion‐neutral frictional heating is shown to play a crucial role in electron density depletion