The Ionospheric Evening Anomalies: Recent Observations and Modeling

Among several intriguing features of the F region ionosphere is the evening enhancement of electron densities over certain mid-latitude regions. The phenomenon takes place during local summer and is most prominent over the regions west of the Antarctic Peninsula during December solstice (known as Weddell Sea Anomaly) and over the Kamchatka Peninsula during June solstice. For these periods and locations electron densities may get up to 2 times larger during the evening/night hours than during noon, while for neighboring mid-latitude locations similar feature is not observed. In order to study the morphology and causes of these anomalies, radio occultation measurements from the six FORMOSAT-3/COSMIC satellites were combined with a physics-based data assimilation model of the ionosphere. The data assimilation model is the Global Assimilation of Ionospheric Measurements Full-Physics model (GAIM-FP), which is based on an Ensemble Kalman filter technique and a physics-based model of the ionosphere/plasmasphere. It covers the altitude range from 90 to 20,000 km, includes six ion species, and allows for inter-hemispheric flow. As an output, the assimilation model provides the 3-dimensional density distribution throughout the ionosphere and information about the physical drivers, including the neutral winds and composition. COSMIC radio occultation data for winter 2008/2009 and summer 2009 were utilized to represent the average global ionosphere. To establish the importance of the various physical drivers, model runs were performed with and without assimilating the data. The 3-dimensonal morphology of the anomalies reproduced by the GAIM-FP was examined and the resulting density, wind and composition were compared. Neutral winds calculated by the model were also compared with HWM93 and HWM07 winds. The results indicate that the thermospheric neutral wind is a main driver of the anomalies but also indicate that the anomalies exhibit a sensitivity to variations in neutral composition and temperature.