The Long-term Trend of Thermospheric Compositions from Whole Atmospheric Simulation and Satellite Observation

JGR space physics:  This study examines the long-term trend of column-integrated atomic oxygen to molecular nitrogen ratio, O/N2, in the upper atmosphere and investigates the cause of this long-term trend in O/N2. 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. O/N2 simulated by NSF NCAR's Whole Atmosphere Community Climate Model 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 O/N2 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 O/N2 of about -0.54% per decade relative to the O/N2 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 O/N2 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 O/N2. The results show that this decreasing trend is mainly caused by the increase in greenhouse gas concentrations.