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

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 stations, with TEC and ROTI used to characterize large‐scale plasma structures and ionospheric irregularities, respectively. These observations are complemented by digital ionosonde measurements to investigate storm‐time F‐region electrodynamics, all‐sky imagers to track airglow depletion evolution, and in situ plasma density observations from the Swarm B satellite to provide independent detection of depletion signatures, while interplanetary and geomagnetic parameters are incorporated to characterize the storm‐time forcing conditions. Prior to storm onset, EPBs appeared as near‐meridional, eastward‐drifting depletions confined to equatorial latitudes. Following storm onset, the EPB structures rapidly intensified, expanded into midlatitudes, and underwent pronounced morphological deformation. Sustained forcing associated with a second ICME coincided with the development of a large‐scale reversed C‐shaped depletion pattern extending into midlatitudes and with zonal drift reversals, particularly over western South America. The storm‐time response exhibited a marked east‐to‐west gradient in deformation, latitudinal extent, and drift perturbations. Prompt penetration electric fields played the dominant role in driving EPB intensification and drift reversal, with their ionospheric effects exhibiting substantial longitudinal variability across the American sector.