Eddy diffusion coefficients scale height

The water photolysis under low O2 pressure always led to a loss of hydrogen into space. The diffusion rate of the H2 (or H after it has been broken down by photolysis) through the homopause and exobase is limited. The definition of the homopause (80-90 km altitude) is the point at which the molecular and eddy diffusion coefficients are equal or, in other words, the critical level below which an atmosphere is well-mixed. The exobase ( 550 km) is the height at which the atmosphere becomes collisionless above that height the mean free path of the molecules exceeds the local scale height (RTIg). 

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