Atmosphere diagram showing the exosphere and other layers. The layers are not to scale: from Earth's surface to the top of the stratosphere (50km) is just under 1% of Earth's radius.

Exobase, also called the critical level, the lowest altitude of the exosphere, is defined in one of two ways:

1. The height above which there are negligible atomic collisions between the particles and
2. The height above which the constituent atoms are on purely ballistic trajectories.

At the exobase, the mean free path of a molecule is equal to one pressure scale height. As the pressure scale height is almost equal to the density scale height of the primary constituent, and since the Knudsen number is the ratio of mean free path and typical density fluctuation scale, this means that the exobase lies in the region where \mathrm{Kn}(h_{EB}) \simeq 1.

References

Gerd W. Prolss: Physics of the Earth's Space Environment: An Introduction. ISBN 3540214267

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