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Formation of the layers of the ionosphere
The grade of ionization depends on the intensity and the wavelength of the incoming solar electromagnetic radiation, as well as the composition of the atmosphere. A significant amount of short-wave solar energy is used to ionize the air in the upper atmosphere.
Photo ionization
The ionospheric layers are formed by photo ionization of atmospheric atoms (X) and molecules (XY). Ionization is mainly due to extreme ultraviolet (EUV) radiation from the Sun. Visible light does not have enough energy to ionize atmospheric gasses. Photo ionization can symbolically be written:
where en is a free electron with low energy, and hí is EUV solar radiation with wavelengths shorter than 134 nm. (i. e., a large part of the ultraviolet (UV) and X-ray band contributes to the ionization.)

For Process 4.2.1 to occur, the photo energy of the EUV-waves must exceed the ionization potential of the constituent X and XY. If we start from the left, the equation above can be read in the following way: An XY molecule in the upper atmosphere is irradiated by emissions from the Sun.

If the energy of this radiation is larger than the ionization potential of XY, an ion pair (XY+ +en) is produced, where en is a free electron with very low energy. An electron was lost during this reaction, and XY+ is a positive ion.

We call en a free electron because it is not longer bound to the neutral molecule. Similar ionization can occur for all gases in the upper atmosphere.
The ionization potential of the dominating atmospheric constituents is shown in table below.
Ions and free electrons are lost by recombination through several possible processes:
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This theme includes:
Structure and Composition of the Middle and Upper Atmosphere
Structure of the upper atmosphere
Dynamic processes in the upper atmosphere
The Ionosphere, - introduction
Electron density in the ionosphere
The layers of the ionosphere
The disturbed ionosphere
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