| | 5 | In [http://iopscience.iop.org/article/10.3847/0004-637X/820/1/3/pdf 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. In [http://iopscience.iop.org/article/10.1088/0004-637X/693/1/23/pdf 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, the exobase is above the sonic point). Thus, H > R,,p,, as well, and therefore greater than mfp of charge exchange near the planet. |
