not-yet-known not-yet-known not-yet-known unknown This study investigates the F-region plasma density and irregularities response during the May-2024 geomagnetic storm using data from Swarm and GRACE-FO missions. While Swarm-B provided measurements at 11/23 LTs, data from GRACE-FO and Swarm-A were collected at 05/17 LTs and 07/19 LTs, respectively. Following the storm commencement on May 10, the equatorial ionization anomaly (EIA) was particularly strong (depleted) on the dayside (nightside). Additionally, an unexpected development of the EIA was observed in the early morning 05/07 LTs. Deviations during the storm’s initial and recovery phases indicate the strong influence of penetration and disturbance dynamo electric fields. Significant equatorial plasma density depletion (EPD) activity with apex altitudes reaching up to ~ 5000 km during pre-midnight on May 11 and ~ 3400 km in the early morning was also noticed. We suggest that the morning EPDs are locally generated rather than being remnants from the previous night.

The strip-like bulge is a storm-time conjugate ionospheric plasma density enhancement that extends widely (over 150° in longitude) in the zonal dimension but occupies only 1°~5° in latitude. Based on in-situ measurements of 11 low earth orbit (LEO) satellites, this study statistically investigates the bulge structures of geomagnetic storms driven by 136 interplanetary coronal mass ejections (ICMEs) during 2000~2021. The statistical results show that the strip-like bulges are initiated at the end of the storm main phase and can persist for more than 60 hours. The spatial and temporal coverage of the strip-like bulge varies from storm to storm. However, the bulges do exhibit occurrence preferences: stronger storms, the Asian-Pacific sector (with eastward magnetic declination), the nightside of the dawn-dusk terminator, and solar minimum periods. A quiet time density enhancement called midlatitude extra peaks could be recognized as a precursor of the strip-like bulge. The plasmaspheric compression shares some similar occurrence features with the strip-like bulge, indicating a field-aligned downward and cross-L inward intrusion of the plasmaspheric ions. The local net ion drifts partly support this scenario with downward/inward being the most dominant but not unique pattern, other diverse net ion drift configurations exist but their impact on the strip-like bulges remains unclear.

This study reports a peculiar nighttime ionospheric enhancement in the total electron content (TEC) from the geostationary (GEO) satellites in response to the geomagnetic superstorm of May 2024. The enhancements occurred at low to mid-latitudes during the storm’s recovery phase on 11-12 May, with the TEC values almost twice as high as the quiet ones. Surprisingly, the nighttime ionospheric enhancements sub-rotated westward with a speed of ~130.5 m/s. The nighttime TEC enhancements lasted ~5-7 hours for a given location; however, they persisted over half a day over wide longitude ranges. Meanwhile, ionosonde data from two equatorial ionization anomaly (EIA) stations showed a higher peak height of the F2 layer, and a significant double-crest EIA structure was observed by both GEO TECs and in situ electron densities from China Seismo-Electromagnetic Satellite during this period of interest, indicating a possible contribution from disturbance electric fields producing nighttime eastward equatorial electric fields. Nevertheless, it remains a mystery whether the nighttime TEC enhancement and its movement were associated with the wind disturbance dynamo.