On the low-latitude ionospheric responses to the May-2024 geomagnetic storm observed by LEO satellites

In this study, we analyze electron density measurements from the Low-Earth Orbiting (LEO) satellite constellations Swarm and GRACE-FO to examine the effects of the May-2024 geomagnetic storm on the equatorial and low-latitude ionosphere. Results show that the equatorial ionization anomaly (EIA) was particularly enhanced on the dayside and depleted on the nightside. Notably, an intensification of the EIA was observed during early morning hours (at ~05/07 LTs) by the GRACE-FO and Swarm A satellites. The observed EIA modifications can be attributed to the strong influence of the electric fields and thermospheric winds. Comparisons with CHAMP and GRACE observations during the Halloween storm indicate an increase of a similar order of magnitude in the EIA’s crest-to-trough ratio (CTR) and L-value around similar local times and longitudes, emphasizing the May-2024 storm as one of the strongest geomagnetic storms in the space age. Additionally, strong equatorial plasma depletion (EPD) activity was noted, including EPDs detected during early morning hours at ~05 LT (~07 LT) by GRACE-FO (Swarm A). These EPDs reached very high apex altitudes of ~5000 km during pre-midnight and ~3400 km at early morning hours during 11-12 May, in contrast to ~1000 km during pre-storm conditions. The lower apex altitude of the early morning EPDs than of the pre-midnight EPDs suggests that these EPDs are generated after midnight and they are not remnants from the previous evening. This suggestion is also supported by ground-based ionosonde observations in Southeast Asia, combined with satellite data, which reveal an elevation of the ionosphere after midnight, supporting the Rayleigh-Taylor instability mechanism crucial for the EPD growth.