This paper presents our effort to assimilate FORMOSAT‐3/COSMIC (F3/C) GPS Occultation Experiment (GOX) observations into the National Center for Atmospheric Research (NCAR) Thermosphere Ionosphere Electrodynamics General Circulation Model (TIE‐GCM) by means of ensemble Kalman filtering (EnKF). The F3/C electron density profiles (EDPs) uniformly distributed around the globe which provide an excellent opportunity to monitor the ionospheric electron density structure. The NCAR TIE‐GCM simulates the Earth's thermosphere and ionosphere by using self‐consistent solutions for the coupled nonlinear equations of hydrodynamics, neutral and ion chemistry, and electrodynamics. The F3/C EDP are combined with the TIE‐GCM simulations by EnKF algorithms implemented in the NCAR Data Assimilation Research Testbed (DART) open‐source community facility to compute the expected value of electron density, which is ‘the best’ estimate of the current ionospheric state. Assimilation analyses obtained with real F3/C electron density profiles are compared with independent ground‐based observations as well as the F3/C profiles themselves. The comparison shows the improvement of the primary ionospheric parameters, such as NmF2 and hmF2. Nevertheless, some unrealistic signatures appearing in the results and high rejection rates of observations due to the applied outlier threshold and quality control are found in the assimilation experiments. This paper further discusses the limitations of the model and the impact of ensemble member creation approaches on the assimilation results, and proposes possible methods to avoid these problems for future work.

[1] This paper presents an application of ensemble Kalman filtering (EnKF) to a general circulation model of the thermosphere and ionosphere. It is designed to incorporate the feedback between plasma and neutral variables in both the analysis and forecast steps of filtering so that thermospheric parameters can be inferred from ionospheric observations and vice versa. We make a case that the global neutral density specification can greatly benefit from this approach based on a number of filtering experiments conducted under the assumption of no model bias. Specific observations considered are (i) neutral mass densities obtained from the accelerometer experiment on board the CHAMP satellite and (ii) electron density profiles obtained from the COSMIC/FORMOSAT-3 mission. Assimilation of the neutral mass density obtained from the CHAMP mission into the TIEGCM is shown to improve the neutral density specification in the vicinity of satellite orbits, but is short of making a global impact unless accompanied by the estimation of the primary driver of the density variability such as solar EUV flux. On the other hand, assimilation of the COSMIC/FORMOSAT-3 electron density profiles into the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM) is far more effective than the CHAMP neutral density in terms of improving the global neutral mass density specification. This suggests a synthesis of thermospheric and ionospheric observations into the general circulation model, brought about with the help of the latest EnKF techniques, can effectively increase the geophysical information content of observations.