Double Auroral Undulation During the Main Phase of the May 10–11, 2024 Storm

We report the discovery of a new type of aurora, namely, double auroral undulation that occurred simultaneously on both equatorward and poleward edges of the duskside auroral oval during the main phase of the May 10–11, 2024 superstorm. Whereas the equatorward auroral undulation (EAU) has been frequently observed and well known, the poleward auroral undulation (PAU) has never been observed yet. Both EAU and PAU were found in the N₂ LBHL (165–180 nm, mostly due to electron precipitation) and Lyman α 121.6 nm (due to proton precipitation) images acquired by the Defense Meteorological Satellite Program (DMSP) spacecraft. The spatial amplitude and wavelength of the PAU reached ∼900 km. During the PAU crossing, DMSP observed both precipitating electrons and ions (protons) and a plasma drift up to ∼5,000 m/s with a strong shear. Simultaneous ground-based all-sky imaging at South Pole captured the dynamic behavior of the PAU that propagated in an anti-sunward direction at a phase speed of ∼250 m/s. The solar wind conditions during the PAU were a high solar wind speed (∼700 km/s), an intense southward interplanetary magnetic field (IMF, approximately −40 nT), and a high density (37 cm⁻³). These upstream conditions suggest that the generation of PAU is likely due to giant Kelvin-Helmholtz waves on the magnetopause for southward IMF.