The Long-Term Trend of Thermospheric Compositions From Whole Atmospheric Simulation and Satellite Observation

This study examines the long-term trend of column-integrated atomic oxygen to molecular nitrogen ratio, $\Sigma O/N_2$, in the upper atmosphere and investigates the cause of this long-term trend in $\Sigma O/N_2$. We first validate the feasibility of using a physics-based model for a long-term climate reanalysis by applying a model-data comparison between 2002 and 2018. $\Sigma O/N_2$ simulated by NSF NCAR's Whole Atmosphere Community Climate Model (WACCM) with thermosphere and ionosphere extension (WACCM-X) and measured by Global Ultraviolet Imager (GUVI) aboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) mission is used to determine the long-term trend of $\Sigma O/N_2$ from 2002 to 2018 and validate the model result. The model and data show good agreement after removing the impact of solar irradiance and geomagnetic activity using a least-squares fitting method, revealing a decreasing trend of $\Sigma O/N_2$ of about $-0.54\%$ per decade relative to the $\Sigma O/N_2$ in 2018 in the model and about $-0.45\%$ per decade in data along the satellite orbit during the period between 2002 and 2018. A decreasing trend of global $\Sigma O/N_2$ of about $-0.70\%$ per decade is found in the model between 1960 and 2018. After that, four WACCM-X long-term simulations are performed from 1960 to 2018 to identify the cause of the decreasing trend of $\Sigma O/N_2$. The results show that this decreasing trend is mainly caused by the increase in greenhouse gas concentrations.