The Role of Downward Plasma Diffusion in the Ionospheric Nighttime Weddell Sea Anomaly

Abstract This study investigates the role of downward plasma diffusion in the ionospheric nighttime Weddell Sea Anomaly (WSA), characterized by an anomalous diurnal electron density peak in nighttime from the South Pacific to South America. Traditionally attributed to prolonged photoionization and ionospheric uplift during summer, the WSA is not reproduced by the latest Thermosphere Ionosphere Electrodynamics General Circulation Model. We introduce a data‐driven flux approach to incorporate more realistic diffusive flux, significantly improving the model's ability to replicate key WSA features. Simulations show that topside plasma diffusion turns downward in the evening, peaking near midnight, contributing notably to the NmF2 enhancement. Around 22:00 local time, downward plasmaspheric diffusion becomes the dominant plasma source, rivaling or exceeding local photoionization (12.08 vs. 8.50 at 22:30). These results offer new insight into the interplay of processes driving the WSA and underscore the importance of plasma diffusion in ionospheric dynamics.