A Simulation Study of NmF2 Semiannual Anomaly at the Zhongshan Station, Antarctica, at Solar Maximum Based on TIEGCM

The semiannual anomaly, characterized by increased peak electron density in the F2 layer (NmF2) at equinoxes compared to solstices, remains incompletely elucidated, especially at high latitudes. The magnetospheric convection pattern introduces additional complexity to its formation mechanisms. This study utilized the NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM) to explore the semiannual anomaly at the Zhongshan Station (ZHS), Antarctica, focusing on the processes occurring in the polar upper atmosphere, which are crucial in the formation of this phenomenon. Simulations reveal that the convective electric fields at high latitudes amplify the semiannual variation of NmF2. Specifically, the daytime peak electron density at ZHS is primarily influenced by the availability of ionization sources at middle-high latitudes for convective transport. During the equinox, this peak is enhanced through transport, due to higher plasma density at middle-high latitudes, whereas in summer, there is a depletion of ionization sources at lower latitudes, results in a less efficient transport effect. The semiannual variation in ionization sources is attributed to changes in the neutral composition driven by thermospheric circulation and neutral temperature. Additionally, during the equinox, the coupling of neutral winds with ion convection draws air parcels with larger O/N₂ from lower latitudes, shaping the distribution of neutral composition into a “neutral tongue,” further intensifying the plasma transport effect. These findings provide new insights into the intricate interactions among magnetospheric, ionospheric, and thermospheric dynamics.