On the Origin of Postmidnight Upwelling Plume‐Like F Region Irregularity Echoes at Low Latitudes on 29 September 2025

Abstract In this study, unusual postmidnight F region irregularities were observed by the Meteor and Ionospheric Irregularity Observation System (MIOS) very high frequency radar at Ledong (18.4°N, 109.0°E) on 29 September 2025 during a moderate geomagnetic storm. The radar echoes exhibited apparent upwelling plume‐like structures after local midnight and persisted for more than 4 hr until near sunrise. Unlike many previously reported postmidnight irregularity events in the East/Southeast Asian sector, this event was accompanied by pronounced global navigation satellite system (GNSS) scintillation. Based on multi‐instrument observations, the origin and development of the postmidnight irregularities and their prolonged duration were investigated. Prior to the appearance of the irregularity echoes, substantial F layer uplift was observed over the equatorial and low latitude regions. While the storm‐time F layer uplift could have provided favorable conditions for the generation of equatorial plasma bubbles (EPBs), multiple observations suggest that these irregularities more likely evolved from preexisting EPB depletion structures drifting eastward from western longitudes. The storm‐time F layer uplift likely reactivated the decaying EPB depletion structures, allowing them to extend to higher altitudes and map to higher latitudes. These reactivated structures, combined with the background ionospheric state, may have contributed to the pronounced GNSS scintillation. Furthermore, the relatively weak zonal drift under storm‐time conditions may have limited the zonal displacement of the irregularity structures, thereby contributing to their prolonged duration within the MIOS radar field of view.