Solar flare-induced gradient drift instability observed by SuperDARN HF radars

A solar flare is a space weather event that causes a transient in the dayside ionospheric system by enhancing plasma density. A solar flare effect (SFE) is typically transient in sub-auroral, middle, and low latitudes dayside ionospheric currents due to changes in the ionospheric electric field and conductivity caused by flare-enhanced photoionization. However, the largest X-class flares can impact current systems at auroral and polar latitudes as well. Using ground-based radars and GNSS TEC receivers located at high and middle latitudes in the North American sector, we analyzed an X9.3 flare on 6 September 2017. We found: (i) the SuperDARN radar located at Saskatoon (at dawn sector) observed a sudden appearance of ionospheric scatter following the flare; (ii) the SuperDARN Inuvik radar, located in the polar cap, recorded a sudden reduction in plasma flow velocity. Near the day-night terminator, SuperDARN radars observed an intensification of irregularities due to the Gradient Drift Instability, GDI. We speculate that flare-enhanced density gradients intensified the conditions for GDI at the day-night terminator. The reduction in plasma flow velocity observed by the Inuvik radar is likely driven by a reduction in the efficiency of mechanical energy conversion in the dayside solar wind-magnetosphere-ionosphere (SW-M-I) interaction.