Thermally Driven Atmospheric Escape: Transition from Diffusion-limited to Drag-off Escape

Abstract We examine the transition from diffusion-limited to drag-off escape using a direct simulation Monte Carlo (DSMC) model developed to simulate planetary atmospheres, referred to as Harrah. A 1D spherically symmetric DSMC model is used to simulate two-component atmospheres to focus on the impact of rarefaction on thermal diffusion. The results for the escape rates and thermal structure are characterized using a mass-averaged Jeans parameter, λ avg,0 , and Knudsen number, Kn a,0 , defined at a reference radial distance r 0 . As a test of the model, we simulated the escape of N 2 and H 2 self-consistently for conditions of Pluto’s upper atmosphere observed during the New Horizons mission. As expected, the DSMC result for H 2 escape is consistent with the diffusion-limited approximation, and H 2 escape has little effect on the background N 2 atmosphere. Then we examined the transition from diffusion-limited to drag-off escape for two-component atmospheres characterized by mass-averaged Knudsen numbers and Jeans parameters of Kn a,0 ≈ 0.01–0.1 and λ avg,0 ≈ 0.5–13, respectively. We found that escape transitioned from hydrodynamic escape to an enhanced Jeans-like escape at λ avg,0 ≈ 3.0–3.6. For values of λ avg,0 < 3.0–3.6, escape was hydrodynamic and in the blow-off regime. For values of λ avg,0 >≈ 3.6, the DSMC results indicated that using the diffusion-limited and drag-off approximations can lead to inaccurate estimates of the escape rate.