Physics
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufmsh43a1153h&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #SH43A-1153
Physics
0328 Exosphere, 0343 Planetary Atmospheres (5210, 5405, 5704), 0560 Numerical Solutions (4255), 6225 Mars, 7837 Neutral Particles (2151)
Scientific paper
Traditionally, exospheric densities and velocity distributions are modelled by spherical symmetric analytical Chamberlain functions, assuming gravity is the only force acting on the neutrals. Planetary exospheres are however not spherical symmetric to any good approximation, as evident from observations, due to effects such as photoionization, radiation pressure, charge exchange, recombination and planetary rotation. To account for these effects numerical simulations are needed. Using Monte Carlo test particle simulations it is possible to account for the above effects (if ion distributions are assumed). Even though neutrals in the exospheres by definition do not collide often, collisions occur. Especially near the exobase the transition is gradual from collision dominated regions at lower heights (with Maxwellian velocity distributions) to essentially collisionless regions at greater heights. We present exospheric simulations that include collisions self consistently using the direct simulation Monte Carlo (DSMC) approach. The code is three dimensional, parallel and uses an adaptive grid, allowing many particles to be included in the simulations, leading to accurate results. In particular, we here study Mars' hydrogen exosphere and the effects of the above processes, including thermal escape rates. Accurate exosphere models are also important for analysis of X-ray and energetic neutral atom (ENA) images, since the measured fluxes are line of sight convolutions of ion fluxes and neutral densities, and asymmetries in the exospheric profiles will directly affect the images. We present implications for observations of solar wind charge exchange X-rays and for observations of ENAs at Mars.
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