Neutral Wind–Electric Field Coupling in the Equatorial Ionosphere During the May 2024 Great storm

The G5-class geomagnetic storm of 10–11 May 2024 produced one of the most extreme space-weather disturbances of Solar Cycle 25, generating large-scale perturbations across the thermosphere–ionosphere system. Over the Indian dip equatorial station Trivandrum (8.5°N, 76.9°E), the storm caused unusually strong enhancements in daytime Vertical Total Electron Content (VTEC) accompanied by distinctive, short-period undulations in electron density. These signatures reveal the strong and often competing roles of storm-time electric fields and thermospheric neutral winds in regulating equatorial plasma dynamics. Understanding their coupled influence is essential for advancing upper-atmosphere physics and improving global space-weather prediction.

In this study, we examine the ionospheric response to the May 2024 Great storm using multi-instrument observations and a physics-based equatorial and low-latitude ionospheric model. Observational datasets include GNSS-derived VTEC, DPS-4D Digisonde electron density profiles and ionospheric electron content (IEC), and high-resolution interplanetary and geomagnetic parameters. Storm-time meridional neutral winds are obtained from AMIE-constrained TIEGCM simulations, while vertical plasma drifts are specified using a prompt-penetration electric field (PPEF) model that maps solar-wind electric fields into the equatorial ionosphere.

To isolate the physical drivers, four controlled model experiments were conducted: (1) quiet-time winds with quiet-time drifts; (2) storm-time PPEF drifts with quiet winds; (3) storm-time winds with quiet-time drifts; and (4) storm-time forcing combining both PPEF and disturbed winds. This approach allows a clear separation of electrodynamic and thermospheric contributions to storm-time plasma redistribution.

The simulations show that PPEF-driven uplift dominates the overall magnitude of the TEC enhancement, raising the F-region peak and increasing the integrated electron content. However, the observed short-period VTEC and density undulations emerge exclusively when storm-time meridional winds are imposed. These winds undergo rapid reversals between poleward and equatorward directions, driven by high-latitude Joule heating and changes in thermospheric circulation. The resulting modulation of field-aligned diffusion produces alternating enhancements and depletions in plasma density, closely matching the temporal structure seen in Digisonde profiles and GNSS VTEC.

The combined PPEF + disturbed wind simulation reproduces the pre-noon features. In the afternoon sector, however, both model and Digisonde underestimate GPS VTEC, indicating a substantial contribution from the plasmasphere above 1000 km, consistent with observed F3 layer signatures. This highlights the importance of including ionosphere–plasmasphere coupling in models aimed at predicting low-latitude storm responses.

Our results provide the first detailed evidence from the Indian sector that rapid meridional wind variability can imprint strong, short-timescale signatures on equatorial electron density during an extreme geomagnetic storm. They demonstrate that neutral winds and electric fields are jointly responsible for shaping storm-time equatorial ionospheric structure, underscoring the need for coupled thermosphere–ionosphere–plasmasphere modeling frameworks.