highionization

High ionization refers to the state of an atom or ion in which a large fraction of its electrons have been removed, resulting in a high charge. In practice, high ionization implies ions with substantial ionization stages (for example, O VII, Fe XVII, or higher in astronomical contexts) and typically corresponds to extreme physical conditions such as very high temperatures or strong radiation fields. It is contrasted with low-ionization gas where few electrons have been removed.

The main mechanisms driving high ionization are collisional ionization in hot plasmas and photoionization by energetic photons. Collisional ionization occurs when electrons gain enough kinetic energy in a hot environment, such as a million-degree plasma, to overcome binding energy and eject electrons. Photoionization occurs when photons with sufficient energy (in the ultraviolet, extreme ultraviolet, or X-ray range) ionize atoms and ions. In many environments, both processes contribute, with the resulting ionization balance depending on temperature, density, radiation field, and chemical composition.

Astronomical observations frequently probe high ionization through spectral lines of highly charged species, seen in ultraviolet and X-ray spectra. The presence of high-ionization lines indicates strong radiation fields or hot gas and is used to diagnose physical conditions like temperature, density, and ionization state. Environments where high ionization is important include active galactic nuclei and their outflows, the warm-hot intergalactic medium, stellar and solar coronae, and laboratory fusion plasmas, where measurements of highly ionized species inform models of energy transport and chemical abundances.