Unusual SuperDARN backscatter during the 11 May 2024 geomagnetic storm

A geomagnetic storm, one of the largest in this solar cycle, was launched on 10 May 2024, producing spectacular auroral displays that could be observed across the continental United States (US) at middle and low latitudes. In this study, we focus on a brief 20-min interval during the peak of the storm when the Sym-H index dropped to ~−500 nT, and the auroral activity specified by the AL and AU indices was elevated. During this interval, the Blackstone (BKS) Super Dual Auroral Radar Network (SuperDARN) radar, observed strong ionospheric backscatter blanketing the near-ranges across its field-of-view. Upon analyzing the elevation and virtual height characteristics of this backscatter we find that: (a) the BKS radar observed F-region backscatter at unusually close ranges (<750 km), and (b) this backscatter was observed over a broad range of elevation angles, including unusual very high ones. It is not physically realistic that all the radio waves, launched over a broad range of elevation angles, refract to become perpendicular to the B-field. We therefore interpret that a sizable portion of this backscatter is produced by irregularities that are not field-aligned. These observations show that plasma irregularities generated during strong geomagnetic storms can produce strong and unusual High Frequency (HF) radar backscatter, and significantly impact their operations. Finally, we suggest that the high-aspect angle backscatter was most likely associated with the non-linear decay of gradient-drift modes that had been excited unusually strongly during the event.