An Extraordinary Dayside Negative Ionospheric Storm of 11 May 2024 and Total Electron Content (TEC) Vortices

On 10–11 May 2024, a major geomagnetic storm occurred on Earth. It significantly affected the dynamics of the Earth's atmosphere and ionosphere. Here we report the occurrence of a severe dayside negative ionospheric storm in the American/Atlantic region during the recovery phase of the 10–11 May 2024 superstorm. The dayside total electron content (TEC) dropped by −90.05 TECU below the quiet‐time values, which represents extreme storm‐time deviations of ∼82%. Our analysis shows that the May 2024 negative deviation was more significant than 9 major storms and superstorms that occurred in 2000–2004, 2015 and 2023–2024. Further analysis revealed that such remarkable TEC deviations were produced by a combination of major decreases in the thermospheric composition and an extreme long‐lasting downward equatorial ExB drift. The recovery phase of the May 2024 storm was also marked by the occurrence of vortex‐like TEC enhancements that resembled the O/N2 column density ratio increases. However, the dynamics of the TEC patches and vortices are very different from that of the O/N2. We also show that TEC vortices can occur during other storms, which means that the ionospheric TEC can serve, to some extent, as a proxy of storm‐time changes in the thermospheric composition.