Global ionospheric specification and forecasting by inferring unobserved thermospheric and ionospheric state variables via an Ensemble Kalman Filter

Chih-Ting Hsu(1), Tomoko Matsuo(2), Wenbin Wang(3), Jann-Yenq Liu(4)

Abstract
This study demonstrates the significance of ion-neutral coupling to ionospheric data assimilation for ionospheric specification and forecast under different seasons and solar activities. Ensemble Kalman Filter (EnKF) is used to assimilate synthetic electron density profiles sampled according to the Formosa Satellite 3/Constellation Observing System for Meteorology, Ionosphere and Climate (FORMSAT-3/COSMIC) into the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM). The combination of the EnKF and first-principles TIEGCM allows a self-consistent treatment of thermosphere and ionosphere coupling in the data assimilation and forecast. Because thermospheric variables affect ionospheric electron densities, different combinations of an observed ionospheric state variable (electron density) and unobserved ionospheric and thermospheric state variables (atomic oxygen ion density, neutral temperature, winds, and composition) are included as part of the EnKF state vector in experiments. Furthermore, these experiments are carried out under different solar activity (F10.7 are 190 and 69, respectively) and season (vernal equinox and summer solstice) to investigate the filter performance under different conditions. In the EnKF, the unobserved state variables are estimated and made dynamically and chemically consistent with the observed state variable, thus improving the performance of the data assimilation system. The impact on ensemble forecast is further examined by initializing the TIEGCM with the assimilation analysis.