= Jeans escape
{c}
Jeans escape is collisionless <atmospheric escape> by particles in the high-speed tail of a thermal distribution at the <exobase>. The <Jeans escape parameter> $\lambda=GMm/(k_BTr_e)$ controls the exponential suppression, and the <Jeans escape flux> is proportional to $(1+\lambda)e^{-\lambda}$. There is no exact positive-temperature threshold below which the tail vanishes. For $\lambda$ of order a few or smaller, a purely hydrostatic tail model can fail and <hydrodynamic atmospheric escape> becomes important.
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