Assessing the Impacts of Assimilating GOLD Disk O/N2 Observations on the Thermosphere-Ionosphere System

The Global-scale Observations of Limb and Disk (GOLD) imagers scan the Earth's Thermosphere-Ionosphere (TI) in the far ultraviolet wavelengths. Measurements from GOLD daylit spectrum are used to retrieve the column integrated atomic oxygen to molecular nitrogen density ratio (O/N₂) over about one fourth of the globe. The present investigation assesses the impact of assimilating GOLD disk O/N₂ on the Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCMX) using the Data Assimilation Research Testbed (DART) ensemble adjustment Kalman filter. Two Observing System Simulation Experiments (OSSEs) are performed, and improvements are quantified by calculating root mean square error (RMSE) and bias with respect to a truth run. In addition to solar and geomagnetic forcing, we introduced gravity wave forcing perturbations to increase the ensemble spread, which has not previously been applied in ensemble assimilation. One of the OSSEs assimilates only the lower atmosphere (LA, 100 km) observations, referred to as LA experiment, and the second assimilates GOLD O/N₂ and LA observations, referred to as Whole Atmosphere (WA) experiment. The WA-analysis O/N₂ RMSE and bias are about 60% and 87% better compared to LA-analysis. Also, the O/N₂ RMSE and bias for the WA-analysis are about 23% and 54% better compared to WA 1-hr forecasts. The improvement in WA electron column density (ECD), a model equivalent of Total Electron Content (TEC), is about 24% compared to the LA experiment. These results demonstrate that the assimilation of GOLD O/N₂ improves both the thermosphere and ionosphere in a whole atmosphere data assimilation system.