Extending the Upper Boundary of Atmospheric Models

A comprehensive model of the whole atmosphere through the exosphere would provide, among other things, improved specification of atmospheric escape, the hydrogen geocorona, plasmasphere refilling following a geomagnetic storm, as well as the probability of collisions faced by resident space objects whose trajectories are perturbed by atmospheric drag. However, most physics-based atmospheric models have an upper boundary that reaches less than 600 km. We propose to extend the upper boundary of TIME-GCM by coupling it to a model of the exosphere.

The coupling of these two domains requires navigating the transition region between continuum fluid dynamics and kinetic particle dynamics. The nearly-collisionless nature of the exosphere is best represented by the direct simulation Monte Carlo (DSMC) method in which individual particle trajectories are propagated through a grid. In contrast, TIME-GCM solves fluid continuity, energy, and momentum equations on pressure surfaces. Here, we describe the adaptation of the DSMC method to an exospheric model and the coupling we perform between the fluid and particle regimes.