Up to about 100km. Some typical values taken from measurments during the 1960s are shown below. In the lower ionosphere the molecular forms of nitrogen and oxygen are most abundant, but above 200km atomic oxygen is the main component. This is significant in explaining the plasma density profile. The different layers in the ionosphere are produced as a result of the differing ability of the solar radiation to ionise the various constituent gases and the ability of the ions and electrons produced to recombine.
Production processes The solar radiation consists of a spectrum of wavelengths.
Because the atmosphere is a mixture of gases these wavelengths are absorbed differently at differing altitudes. Knowing the gas concentration as a function of alititude it is possible to calculate the optical depth of the atmosphere for different wavelengths. Wavelengths between about 15-80 nm. The rest of the radiation penetrates lower into the atmosphere. Two bands on either side of this from a few nm.