Modeling and analysis of ionospheric evening anomalies with a physics-based data assimilation model

• Data assimilation models are used to study evening anomalies.
• TWAM winds closely reproduce NmF2 and hmF2 over the evening anomaly sites.
• Geographic meridional wind drives the anomalies in both hemispheres.
• Zonal wind is important to provide close agreement with COSMIC data over the WSA.
• Transport process was found to play a critical role in the generation of anomalies.

Anomalous evening enhancements of electron densities in the mid-latitude ionosphere take place during summer and are most prominent over the west of the Antarctic Peninsula (Weddell Sea Anomaly). Although the phenomenon has been known for several decades, its generation mechanism is still being debated and its modeling remains a challenge. In this paper, data assimilation models were used to understand the role of thermospheric winds in the anomalies, and to elucidate the physical mechanism behind them. COSMIC radio occultation data were used and a newly developed Thermospheric Wind Assimilation Model (TWAM) was employed to estimate the horizontal wind components. Next, the TWAM winds were used to drive the Ionosphere-Plasmasphere Model to simulate the anomalies. The model results show close quantitative agreement with the COSMIC measurements and indicate that while the geographic meridional wind alone can drive the electron density evening peak, the zonal wind further enhances the anomaly. Furthermore, for closer agreement with the COSMIC data, the zonal wind effect was found to be important. To understand the physical mechanism behind the anomalies, the plasma production, loss and transport processes were analyzed. It was found that due to the equatorward wind during the evening, the density maximum forms at higher altitudes where the density reduction due to recombination is slow. Furthermore, it was revealed that during the evening, the plasma loss due to transport weakens. As a consequence of the reduced rate of recombination and the weakened plasma loss due to transport, the relative role of solar production increases and the electron density enhancement occurs.