Simulating Transport and Escape with a Coupled Fluid-Particle Atmospheric Model

At altitudes approaching the exobase, the assumptions used in fluid approximations of atmospheric dynamics begin to break down. This behavior is the primary reason that such models do not extend higher than approximately 400-600 km. This barrier can be overcome by transitioning to a direct simulation Monte Carlo (DSMC) particle model formulation near and above the exobase.

The upper boundary of fluid models can be modified to accept conditions imposed by a DSMC model of the exosphere. Similarly, DSMC models can accept input from a fluid model at its lower boundary. We have successfully performed a two-way coupling between two such models, using TIME-GCM to simulate the thermosphere and MONACO to simulate the exosphere. This leads to a cohesive fluid / particle model that extends from approximately 30km to 20,000km.

Here, we discuss how the upper boundary of TIME-GCM has changed due to the introduction of the particle exosphere model. We also show how the distribution of hydrogen in both models has been affected by the two-way coupling. Finally, we discuss the escape flux through the upper boundary of the exosphere model and how it compares to Jean’s escape as well as its relation to the limiting flux.