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, , in the upper atmosphere and investigates the cause of this long‐term trend in . 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. 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 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 of about per decade relative to the in 2018 in the model and about per decade in data along the satellite orbit during the period between 2002 and 2018. A decreasing trend of global of about 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 . The results show that this decreasing trend is mainly caused by the increase in greenhouse gas concentrations.