Ionospheric Mechanisms Leading to Plasma-Bubble Reversal and Deformation During the 12 November 2025 Geomagnetic Storm

In this study, we present a multi-instrument analysis of the ionospheric response to the intense geomagnetic storm of 12 November 2025 over the American sector, with emphasis on the longitudinally asymmetric deformation, latitudinal expansion, and zonal drift reversal of Equatorial Plasma Bubbles (EPBs). Our analysis integrates Total Electron Content (TEC) and Rate of TEC Index (ROTI) maps derived from more than 2,800 Global Navigation Satellite System (GNSS) stations, together with coordinated measurements from digital ionosondes, all-sky imagers, and in-situ plasma density observations from the Swarm B satellite. Energetic particle precipitation data from the MetOp satellites, along with interplanetary and geomagnetic parameters, are also incorporated. Prior to storm onset, EPBs appeared as near-meridional, eastward-drifting depletions confined to equatorial latitudes. Following the arrival of an interplanetary coronal mass ejection (ICME) shock, the structures rapidly intensified, expanded into midlatitudes, and underwent pronounced morphological deformation. Sustained forcing associated with a second ICME during the storm main phase coincided with the development of midlatitude reversed C-shaped EPBs and clear nighttime zonal drift reversals, particularly at western South American longitudes. The storm-time response was strongly longitudinally organized, exhibiting a marked east-to-west gradient in deformation, latitudinal extent, and drift perturbations. Prompt penetration electric fields (PPEFs) played the dominant role in driving EPB intensification and drift reversal, while enhanced energetic particle precipitation near the South Atlantic Magnetic Anomaly (SAMA) likely modulated the disturbed electrodynamic response. These observations highlight the sensitivity of EPB dynamics to transient magnetospheric electric fields during extreme geomagnetic conditions.