Abnormal Ionospheric Storm Phases and Significant Equatorial Plasma Bubbles Over the Asian Sector During the 16 April 2025 Geomagnetic Storm

This study investigates the storm‐time response of the low‐latitude ionosphere in the Asian sector and the spatiotemporal evolution of post‐sunset equatorial plasma bubbles (EPBs) during the intense geomagnetic storm on 16 April 2025. Based on data from the Chinese Meridian Project (dual‐channel optical interferometers, Sanya Incoherent scatter radar, and GNSS receivers), multiple complementary data sets (GUVI/TIMED, Swarm, and COSMIC‐2), a comprehensive analysis was conducted. The results show an abnormal “negative‐then‐positive” storm phase response in the low‐latitude ionosphere, alongside the development of post‐sunset EPBs. The main findings are summarized as follows. (a) On 16 April, the low‐latitude ionosphere over China and adjacent regions first exhibited a pronounced negative storm, with decreases exceeding 20 TECU relative to the quiet‐time level, before transitioning to a positive storm during the recovery phase, with enhancements of 15–20 TECU above the quiet‐time background. This temporal evolution is distinctly different from the commonly observed “positive‐then‐negative” two‐stage response; (b) Significant post‐sunset EPBs were observed in the Asian sector during 13:00–23:30 UT on 16 April. These strong EPBs extended to ∼30° MLAT, showed significant westward drift, and persisted for 10 hr throughout the night. These abnormal low‐latitude ionospheric disturbances and distinct irregularity structures could be largely attributed to the storm‐time electrodynamic effect due to electric field perturbations, along with contributions from the composition effect. The findings of this study provide critical evidence elucidating key magnetosphere‐ionosphere‐thermosphere coupling processes and advance our understanding of storm‐time dynamics in the low‐latitude ionosphere.