Benedicte D. Stewart

Parallel 3D Hybrid Continuum/DSMC Method for Unsteady Expansions Into a Vacuum

We present the application of a unidirectional unsteady coupling between a continuum solver and a three dimensional parallel Direct Simulation Monte Carlo (DSMC) code. Two different problems have been considered: the spherically symmetric expansion of a water vapor cloud into a vacuum and the late stages of a comet impact on the Moon. In both cases, unsteady data pre-computed from a continuum solution are used as input to the DSMC code at a fixed interface. The DSMC results were then compared to the continuum results downstream of the interface in a region of mutual validity in order to validate our approach. The DSMC results for the expansion flow showed good agreement with the analytic solution for the density, velocity and temperature downstream of the interface. Similarly, for the comet impact simulations, the DSMC density agrees well with the solution from the continuum solver, the SOVA hydrocode. A slightly hotter solution is however obtained downstream of the interface using the DSMC code compared to the hydrocode solution.

Simulation Of Plasma Interaction With Io's Atmosphere

One dimensional Direct Simulation Monte Carlo (DSMC) simulations are used to examine the interaction of the jovian plasma torus with Io’s sublimation atmosphere. The hot plasma sweeps past Io at ∼57 km/s due to the external Jovian magnetic and corotational electric fields and the resultant energetic collisions both heat and dissociate the neutral gas creating an inflated, mixed atmosphere of SO2 and its daughter products. The vertical structure and composition of the atmosphere is important for understanding Io’s mass loading of the plasma torus, electron excited aurora, and Io’s global gas dynamics. Our 1D simulations above a fixed location on the surface of Io allows the O+ and S+ ions to drift down into the domain where they then undergo elastic and charge exchange collisions with the neutral gas. Each electron’s position is determined by the motion of a corresponding ion; however, the electrons retain their own velocity components which are then used during elastic, ionization, and excitation collisio...

Io’s Atmospheric Freeze‐out Dynamics in the Presence of a Non‐condensable Species

One dimensional direct simulation Monte Carlo (DSMC) simulations are used to examine the effect of a trace non‐condensable species on the freeze‐out dynamics of Io’s sulfur dioxide sublimation atmosphere during eclipse and egress. Due to finite ballistic times, essentially no collapse occurs during the first 10 minutes of eclipse at altitudes above ∼100 km, and hence immediately after ingress auroral emission morphology above 100 km should resemble that of the immediate pre‐eclipse state. In the absence of a non‐condensable species the sublimation SO2 atmosphere will freeze‐out (collapse) during eclipse as the surface temperature drops. However, rapid collapse is prevented by the presence of even a small amount of a perfect non‐condensable species due to the formation of a static diffusion layer several mean free paths thick near the surface. The higher the non‐condensable mole fraction, the longer the collapse time. The effect of a weakly condensable gas species (non‐zero sticking/reaction coefficient) w...

Modeling Io’s Sublimation‐Driven Atmosphere: Gas Dynamics and Radiation Emission

Io’s sublimation‐driven atmosphere is modeled using the direct simulation Monte Carlo method. These rarefied gas dynamics simulations improve upon earlier models by using a three‐dimensional domain encompassing the entire planet computed in parallel. The effects of plasma impact heating, planetary rotation, and inhomogeneous surface frost are investigated. Circumplanetary flow is predicted to develop from the warm subsolar region toward the colder night‐side. The non‐equilibrium thermal structure of the atmosphere, including vibrational and rotational temperatures, is also presented. Io’s rotation leads to an asymmetric surface temperature distribution which is found to strengthen circumplanetary flow near the dusk terminator. Plasma heating is found to significantly inflate the atmosphere on both day‐ and night‐sides. The plasma energy flux also causes high temperatures at high altitudes but permits relatively cooler temperatures at low altitudes near the dense subsolar point due to plasma energy depleti...