MONTE-CARLO SIMULATION OF THE TERRESTRIAL HYDROGEN EXOSPHERE

Methods for Monte Carlo simulation of planetary exospheres have evolve d from early work on the lunar atmosphere, where the regolith surface provides a well defined exobase. A major limitation of the successor s imulations of the exospheres of Earth and Venus is the use of an exoba se surface as an artifice to separate the collisional processes of the thermosphere from a collisionless exosphere. In this paper a new gene ralized approach to exosphere simulation is described, wherein the exo base is replaced by a barometric depletion of the major constituents o f the thermosphere. Exospheric atoms in the thermosphere-exosphere tra nsition region, and in the outer exosphere as well, travel in ballisti c trajectories that are interrupted by collisions with the background gas, and by charge exchange interactions with ionospheric particles. T he modified simulator has been applied to the terrestrial hydrogen exo sphere problem, using velocity dependent differential cross sections t o provide statistically correct collisional scattering in H-O and H-H interactions. Global models are presented for both solstice and equin ox over the effective solar cycle range of the F-10.7 index (80 to 230 ). Simulation results show significant differences with previous terre strial exosphere models, as well as with the H distributions of the MS IS-86 thermosphere model.