Thermally-driven atmospheric escape: Transition from hydrodynamic to Jeans escape

Understand

Thermally-driven atmospheric escape evolves from an organized outflow (hydrodynamic escape) to escape on a molecule by molecules basis (Jeans escape) with increasing Jeans parameter, the ratio of the gravitational to thermal energy of molecules in a planet's atmosphere.

• This transition is described here using the direct simulation Monte Carlo method for a single component spherically symmetric atmosphere.
• When the heating is predominantly below the lower boundary of the simulation region, R0, and well below the exobase, this transition is shown to occur over a surprisingly narrow range of Jeans parameters evaluated at R0: {\lambda}0 ~ 2-3.
• The Jeans parameter {\lambda}0 ~ 2.1 roughly corresponds to the upper limit for isentropic, supersonic outflow and for {\lambda}0 >3 escape occurs on a molecule by molecule basis.