Morphology and Causes of the Weddell Sea Anomaly

One of the most outstanding feature of the F region ionosphere is the Weddell Sea Anomaly (WSA), a phenomenon in which the electron densities are larger at night than during the day for periods of southern hemisphere summer months. In order to study the morphology and causes of the WSA, radio occultation (RO) 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). The model is based on an Ensemble Kalman filter technique and a physics-based model of the ionosphere/plasmasphere (IPM). which covers the altitude range from 90 to 20,000 km, includes six ion species (NO+, O2+, N2+, O+, H+, He+), is based on the International Geomagnetic Reference Field (IGRF), and allows for inter-hemisphere 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, composition and electric fields. Model runs were performed with and without assimilating the COSMIC data for two 60-day periods centered around the 2007 and 2008 December solstices, when the WSA is known to be most prominent. The 3-dimensonal morphology of the WSA reproduced by the assimilation model was examined and the resulting density, wind, composition, and electric field pattern were compared. Quantitative results will be given that establish the importance of the various physical drivers.