Unusual equatorial plasma depletions embedded in background ionospheric enhancement related with energetic electrons during March 31, 2001, geomagnetic storm

During the recovery phase of March 31, 2001, geomagnetic storm, post-midnight equatorial plasma depletions (EPDs) associated with abnormal background ion density enhancement were identified by ROCSAT-1 satellite near 180°E. The ion vertical drift within the EPDs was downward and shared a similar pattern to the reference days. Together with the Thermosphere–Ionosphere Electrodynamics General Circulation Model (TIEGCM) simulation, we inferred that traditional cognitions of the electrodynamics and neutral disturbance were not able to explain either the background density enhancement or the EPDs. With the same temporal and spatial coverage, the Polar-orbiting Operational Environmental Satellite (POES) observed significant energetic electron flux enhancement. We suggest that abundant quasi-trapped electrons that originated from inner radiation belt might be responsible for the abnormal ion density enhancement and further facilitate the occurrence of post-midnight EPDs at the topside ionosphere.