Solar Flare-Induced Gradient Drift Instability Observed by SuperDARN HF Radars

Solar flares are a rapid increase in solar irradiance, specifically in X-ray and Extreme Ultraviolet spectra, which enhances the ionization in the dayside ionosphere and creates Sudden Ionospheric Disturbances (SIDs). SIDs are known to create space weather impacts on traveling high frequency (HF: 3–30 MHz) radio waves, by disrupting the communication channels. In this study, we examine ionospheric scatters at dawn terminator, which stems from a severe X9.3 flare on 6 September 2017 peaked at 12:02 UT, utilizing SuperDARN HF coherent scatter radars and Global Navigation Satellite System (GNSS) Total Electron Content (TEC) observations. Specifically, we are interested in the transients in the ionospheric electrodynamics at the sub-auroral latitude near the terminator stemming from the flare effect. Observations suggest that flare-induced density gradient likely favors the formation of gradient-drift instability near the dawn terminator, leading to the irregularities observed by the SuperDARN radars with line-of-sight (LoS) Doppler velocity reaching nearly 300 m/s. The flare amplifies the eastward TEC gradient near the dawn terminator by approximately 2–3 times compared to a geomagnetically quiet and non-flare day. The observed irregularities, attributed to flare-driven instabilities, exhibit a velocity consistent with the equatorial return flow of ionospheric Hall convection. In contrast to prior studies indicating decreased cross-polar-cap potential and associated ionospheric convection flow, our findings show the flare is followed by an increase in localized electric field near the dawn terminator, as depicted in radar LoS velocity.