The ionosphere is created by the ionization of the neutral atoms and molecules of the atmosphere. There are two basic requirements for the formation of an ionosphere: (1) the presence of a neutral atmosphere, and (2) a source of ionization for these gases.
There are two basic sources of ionization: (1) photons and (2) energetic particles. The former process is called photoionization while the latter is called ``impact ionization''. The photons come primarily from the Sun (bremsstrahlung from precipitating particles is also sometimes important) while the ionizing particles can be cosmic rays, or electrons or ions from the Sun, magnetosphere, or a different region of the ionosphere.
The only requirement on the ionizing photons or particles is that their energies ( or , respectively) must exceed the ionization potential or binding energy of the atom or molecule. In most cases EUV and UV solar photons with nm produce the dayside ionospheres of most planets. However, electron impact ionization is very important for Io's ionosphere [Li, 1992] and for ionization at auroral and polar cap latitudes at Earth, particularly during space weather events.
Ionization sources and losses determine the detailed structure of ionospheres. Accordingly, details of multiple chemical reactions, photodissociation, and impact ionization processes must be considered for the sources, while both recombination and chemical reactions must be considered for the losses.