https://en.wikipedia.org/wiki/Thermosphere
The highly diluted gas in this layer can reach 2,500 °C (4,530 °F) during the day. The mass of the thermosphere above about 85 km is only 0.002% of the total mass. it is so near vacuum that there is not enough contact with the few atoms of gas to transfer much heat.
Could Air in Thermosphere Treated as Continuum? @@
Try to calculate Knudsen number:
Kn=kb*T/(2^0.5*pi*d2*p*L)
kb=1.38*10−23 J/K
T=2773 K
d=4*10-10 m
p=0.1 Pa
and the Kn is around 0.5383. Is it OK to treat the air as continuum in Thermosphere?
Thank you!
An extremely large volume of air in the thermosphere, say a cube of length L on a side and volume L^3, where L is very much larger than the mean free path (which is itself larger than the average distance between air molecules) could for some purposes be treated as a continuum. For example, sound can propagate through any gas, however rarefied, approximated as a continuum phenomenon, if the wavelength of the sound greatly exceeds the average distance between gas molecules. Of course, the energy carried by such sound waves would be minuscule, as the gas density is minuscule, but they nevertheless can still exist.
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