Structure and evolution of a quasi-collisional gas torus (case of the Triton hydrogen torus)

Quite generally, the atmosphere of a planet has an escaping component that forms a gas cloud in its circumplanetary space. This gas cloud is frequently detectable and its observation provides important information about both the source and the circumplanetary environment. As a result, there is a strong motivation to investigate the nature of such gas clouds. Up to now, attention has been directed almost exclusively to the case of collisionless clouds, i.e., the collision time is much longer than the lifetime of the cloud constituents. In this study, we consider the relatively unexplored case of a quasi-collisional cloud, i.e., the self-collision time is much longer than the orbital period and much shorter than the lifetime of the cloud constituents. A slightly modified version of the traditional DSMC approach is applied to the case of the hydrogen torus of Triton, which is an example of a spatially axisymmetric quasi-collisional cloud that is generated from a source orbiting a central planet. A large number of calculations were performed for different models, primarily with different source velocity distributions. The results for two models that bracket the full span of these calculations are presented in detail and compared. The spatial structure and temporal evolution is discussed for these systems. The effect of lifetime processes and radiation pressure is also discussed.