A balance achieved in a hot gas when the number of ionizations of a particular ion equals the number of recombinations into that ion. An example of a gas in ionization equilibrium is the Sun’s corona. Thus, in ionization equilibrium, the number of ionizations of the ion Fe⁹⁺, which emits the red coronal line, to the Fe¹⁰⁺ ion equals the number of recombinations from Fe¹⁰⁺ to give Fe⁹⁺.