The thermosphere: a part of the heterosphere

By J. Vercheval

The heterosphere is the atmospheric region where the composition is not homogeneous. The transition with the homosphere is located on the average at about the 100 km altitude where turbulent mixing is no more sufficient to maintain the air homogeneous. Molecular diffusion becomes the main phenomena. The number density of each atmospheric constituent decreases with height at a rate which is directly proportional to its mass: the concentration of heavy constituents (O2 and N2) decrease faster than that of light constituents (O, He and H) in such a way that the relative abundances of the latter continuously increase with height. On going upward, one meet several belts where molecular nitrogen, atomic oxygen, helium and hydrogen successively become the main constituent. The boundaries of these belts vary with the temperature linked itself to the level of solar activity.