A Tale of Two Hemispheres: Asymmetric Geospace Responses and the GNSS Paradox of the October 2024 Severe Geomagnetic Storm

Space weather events triggered by solar activity impact critical technologies like the Global Navigation Satellite System (GNSS) by causing atmospheric imbalances that alter ionospheric electron density. This study investigates the geospace response to the severe geomagnetic storms of October 2024, focusing on the coupling and compositional exchange between the ionosphere and thermosphere. Data were analyzed from two near-magnetic conjugate mid-latitude African stations, Rabat (RABT) and Hermanus (HNUS), using GNSS-TEC measurements alongside thermospheric circulation observations from NASA GOLD and solar wind indices from OMNIWeb. The October 2024 storm, which reached a minimum Dst of -333 nT, drove a negative ionospheric storm phase marked by Total Electron Content (TEC) depletions exceeding 50 TECU. This response was driven by storm-time thermospheric upwelling of N2-rich air, which lowered the O/N2 ratio and accelerated plasma loss via charge-exchange reactions. Furthermore, a distinct hemispheric asymmetry was observed, as the equatorward thermospheric circulation in the Northern Hemisphere arrived before that of the Southern Hemisphere. Direct post-processing of ECEF coordinates using RTKLIB revealed a "GNSS Paradox": while positioning accuracy significantly degraded at HNUS with errors increasing by up to 270%, it counterintuitively improved at RABT, where errors reached their minimum during the main and early recovery phases of the storm. These findings highlight that the technological impact of severe space weather is determined not just by storm magnitude, but by the specific sign and spatial structure of the regional ionospheric response.