GNSS Analysis of the Ionospheric Response to the Extreme Geomagnetic Storm of May 11, 2024: Latitudinal Dependence of VTEC

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Résumé

This study examines the ionospheric response to the extreme geomagnetic storm of May 11, 2024, (Dst ≈ −400 nT) using GNSS derived vertical total electron content (VTEC) from four stations distributed from northern mid-latitudes to southern low latitude. Disturbances are quantified using the relative deviation parameter (ΔVTEC) and interpreted together with GUVI/TIMED thermospheric O/N2 ratio maps. The results highlight a strong latitudinal dependence and a clear spatiotemporal evolution. At northern mid-latitudes, intense and prolonged negative storms (down to −60%) developed mainly during recovery, consistent with storm-induced thermospheric composition changes (O/N2 depletion). At the equatorial station, rapid alternations between negative and positive phases indicate dominant electrodynamic forcing through prompt penetration and disturbance dynamo electric fields. In contrast, the southern low-latitude station experienced extremely intense positive storms, including an exceptional +400% peak during the main phase and persistent enhancements during recovery. A clear latitudinal latency is identified, supporting a two-stage response: rapid electrodynamic forcing followed by delayed equatorward propagation of thermospheric composition disturbances. These findings highlight the coupled role of electrodynamic and thermospheric processes in extreme storm-time ionospheric variability affecting GNSS systems.

Mots-clés

ionosphere; extreme geomagnetic storm; VTEC; ΔVTEC; GNSS; latitudinal dependence