A Coupled Thermosphere-Exosphere Model: Results and Implications for Hydrogen Transport

As the atmosphere transitions from the thermosphere to the exosphere, the changing dynamics of the gas necessitate a change in the physics used to model this region. Above the exobase, which resides roughly at 500km, the gas can be modeled with a direct simulation Monte Carlo (DSMC) model using individual particle trajectories; below, the gas can be modeled using fluid dynamics. This work examines the results of a coupled fluid-particle model that simulates the atmosphere from the mesosphere (~32km) through the exosphere (~20,000km). The DSMC component of the model elucidates how thermospheric and exospheric hydrogen circulation compete and complement each other. Additionally, it is shown that using the DSMC model as an upper boundary condition for hydrogen escape in the fluid model results in a change in the density of hydrogen throughout the upper thermosphere, all the way down to the turbopause. Finally, implications for analytical exospheric density profiles are discussed.