Large-scale ionospheric hole, unusual ionospheric gradients and irregularities observed over South America during the May 2024 geomagnetic superstorm

One of the most powerful geomagnetic storms in several decades hits the Earth on 10–11 May 2024. Using ground-based GNSS (Global Navigation Satellite Systems) observations, Swarm and GOLD satellite data, we investigated the dynamic changes of the equatorial ionosphere in the American sector during the main phase of this superstorm. A dramatic intensification of the equatorial ionization anomaly (EIA) was registered after ∼22 UT on 10 May 2024, associated with eastward prompt penetration electric field effects in the dusk sector over South America. The Swarm–A satellite overflying the American sector just after sunset observed the EIA crest-to-crest distance reaching ∼60° in latitude extent—among the largest ever recorded since the October 2003 superstorm. The extensively wide and deep EIA trough, covering 20–30° in latitude over the equatorial region, featured also plasma bite-out effects in Swarm in situ observations. In addition, the southern EIA crest experienced a remarkable transformation into a “V”-shaped structure, where its right-side wing appeared as a very narrow, stretched band of high ionospheric density along eastern coast of South America. The ground-based GNSS ROTI (Rate of TEC change Index) data revealed formation of a peculiar, tilted pattern of ROTI detections at midlatitudes in South America, significantly different from typical equatorial plasma bubble (EPB) observations. Our analysis demonstrates a clear overlap between the titled structure of high ROTI values at South America midlatitudes and localized ionospheric enhancement inside the narrow right-side wing of the V-shaped southern EIA crest. Steep TEC gradients formed at the equatorward and poleward walls of the modified southern EIA crest due to low TEC values in the deep EIA trough and high TEC values within the crest’s narrow structure; this resulted in high ROTI values over this area. Analysis of GNSS ROTI, GNSS TEC, GOLD UV, and scintillation receiver data did not provide solid observational evidence of storm-intensified development of EPBs. The May 2024 storm offers a rare opportunity to observe large-scale plasma bite-out over continental South America, allowing detailed investigation of the storm-time EIA intensification and peculiar EIA crests/trough transformation.