Ionospheric responses to the May 2024 geomagnetic storm over the East‑Southeast Asia and Australia

This study utilizes GPS‑derived total electron content (TEC), critical frequency (foF2) from ionosondes, TEC from Swarm satellite measurements, the [O]/[N2] ratio from TIMED/GUVI, and the vertical electron density profiles from COSMIC to examine the ionospheric response over the East‑Southeast Asia and Australian sector during the May 2024 magnetic storm, the strongest since November 2003. The storm began at 17:05 UT on May 10, marked by a Sudden Storm Commencement (SSC) with an amplitude of approximately 74 nT in the SYM‑H index, which rapidly declined to about –518 nT by 02:14 UT on May 11. GNSS receiver chains were used to study the morphology of the equatorial ionization anomaly (EIA) during the entire storm period. During the main phase, a negative ionospheric storm effect was observed, as indicated by the decrease in foF2 at the PHUT and BACL stations and TEC at both of hemispheres. The decrease in foF2 and TEC is believed to be caused by the impact of the prompt penetration electric field (PPEF). A complex evolution of the EIA morphology was observed during the recovery phase. During the recovery phase on May 11, hmF2 exhibited repeated uplifts over Southeast Asia in response to fluctuating PPEF associated with successive southward‑to‑northward and northward‑to‑southward reversals of the interplanetary magnetic field (IMF) Bz before 15:00 UT. On May 11, the EIA structure showed degradation, with a double‑peak morphology in both hemispheres associated with strong PPEF fluctuations. The EIA crests shifted equatorward, and an enhancement/reduction of foF2 at the equator/near EIA crest region in the Northern Hemisphere was observed, which is associated with PPEF, equatorward meridional winds and changes in [O]/[N2] ratio. The effect of the ionospheric disturbance dynamo was identified from magnetic data at PHUT and GUAM observatories. The disappearance of the northern EIA crest observed on May 12 might have resulted from the combined influence of factors such as the ionospheric disturbance dynamo effect, meridional winds (the equatorward meridional and transequatorial meridional winds), and the decrease in the [O]/[N2] density ratio. Hemispheric asymmetry of the EIA was observed during the first two days of the recovery phase (May 11‑12). On May 13, an increase in the amplitude of the southern crest (14 TECU) was observed compared to May 12; the northern crest reappeared, and both crests showed a tendency to shift polewards.