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Ionization & Radiation Pressure

Fluid Approximation of Charge Exchange

In ​Christie et al., they calculate the mean free path (sound speed over rate of reaction) for charge exchange and note that it is less than the planetary radius (in regions dominated by the planetary wind) to justify using the fluid approximation, with mixing and exchange primarily at turbulent boundaries. This carries an assumption that the hot and cold hydrogen have the same temperature, but changing the temp only changes the mean free path by a factor of sqrt(2) at most. In ​Murray-Clay et al., they justify the fluid approximation in general by comparing the scale height H to the mean free path of a particle, with the fluid approximation holding for H > lambda_mfp at the sonic point (in other words, when the exobase is above the sonic point).