Assimilation of FORMOSAT‐3/COSMIC electron density profiles into a coupled thermosphere/ionosphere model using ensemble Kalman filtering

Abstract

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.

On the possible use of radio occultation middle latitude electron density profiles to retrieve thermospheric parameters

“…This assumption, in cases with large NmF2 values, can result in either positive or negative errors larger than 10 5 cm À3 at the bottom of the retrieved profiles (Syndergaard et al 2006). This conclusion seems to support the results by Lee et al (2012) who have attempted to assimilate FORMOSAT-3/COSMIC GPS occultation observations into the NCAR TIEGC model. A comparison of the assimilation results with the independent ionosonde observations has shown the improvement of the primary ionospheric parameter, such as NmF2 and hmF2, however 35% of RO EDP were rejected mainly due to either outlier threshold or quality control.…”

“…The coincidence in the bottom part of EDP is not good for the 40% of the analyzed cases, and this may be a serious limitation for our method. This is close to the estimate by Lee et al (2012) who rejected about 35% of IRO EDP due to either outlier threshold or quality control.…”

“…Lee et al (2012) have also reported large percentage (35%) of rejected FORMOSAT-3/COSMIC RO profiles.…”

“…Recent studies show that ionospheric electron densities derived from the RO sounding around and above the F2 peak are reasonably accurate, whereas those below the F region should be used with great caution because of the assumption of spherical symmetry used in the Abel inversion (Lei et al 2007;Kelley et al 2009;Liu et al 2010;Yue et al 2010). As a result, Lee et al (2012) apply a C/F3 EDP observation error assumed to be the sum of a 10% instrumentation error and the estimated Abel inversion error percentage (Liu et al 2010;Yue et al 2010). …”

The impact of FORMOSAT-5/AIP observations on the ionospheric space weather

“…This approach has been applied to assimilate thermospheric/ionospheric observations into a three-dimensional global thermosphereionosphere coupled model, National Center for Atmospheric Research Thermosphere-Ionosphere-Electrodynamics General Circulation Model (NCAR TIE-GCM) (Matsuo and Araujo-Pradere 2011;Lee et al 2012Lee et al , 2013Chartier et al 2013Chartier et al , 2016Hsu et al 2014;Chen et al 2016). Chen et al (2016) successfully constructed a module capable of assimilating ground-based GPS total electron content (TEC) observations to this ionospheric data assimilation system.…”