## R

where v is the upward velocity of a molecule of mass m. The critical level for collisions, which we now call the exobase, falls between 500 and 600 km altitude on Earth. It occurs when the vertically integrated density accounts for one mean free path, i.e. at radius rc from the centre of the planet where

r c where N is the number density, a is the collisional cross-section, H is the scale height, and rc defines the height of the exobase.

The Jeans' escape flux Fjeans (mol cm~2 s_1) is obtained by integrating upwards from this level and over the solid angle, assuming isothermal conditions and a Maxwellian distribution. The factor 1/2 allows for depletion of the fast tail of the distribution by the escape itself.

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