Exosphere

The exosphere is the dilute outer atmospheric region in which a particle can travel a substantial distance without a collision. Its lower boundary, the exobase, is near equality of the mean free path and the local atmospheric scale height. Escaping and gravitationally bound trajectories can coexist.

The exobase marks the transition between a collisional atmosphere and its exosphere. For a neutral, approximately hydrostatic ideal gas, take mean free path $\ell \simeq 1/(n{\sigma }_c)$ and atmospheric scale height $H=k_{B}T/(mg)$. The criterion $\ell \sim H$ givesAn order-one collision convention can change this coefficient. The pressure is not universal: the collision cross-section, composition, local gravity, and ionization must be specified. A strongly escaping, non-hydrostatic atmosphere requires an appropriate density-gradient length instead of a hydrostatic $H$.