CoronalEquilibrium

Coronal equilibrium is a term used in solar and stellar corona physics to describe a steady-state ionization balance in a hot, tenuous plasma at a given electron temperature and density. In this state, the rates of ionization and recombination for each ionization stage balance over time, so the ion populations are determined by the local conditions rather than by transient processes.

Under typical coronal conditions, the plasma is optically thin, electrons follow a Maxwellian distribution, and photoionization by external radiation is negligible compared with collisional processes. The populations Ni of each ionization stage evolve according to a set of coupled rate equations that include collisional ionization from stage i to i+1 and recombination from i+1 back to i. In steady state, dNi/dt = 0 for all i, so the ion fractions fi = Ni/Ntot depend primarily on the electron temperature Te (and, to a lesser extent, on electron density ne). A common simplifying relation in adjacent stages is Ni+1/Ni ≈ qi/αi+1, where qi is the collisional ionization coefficient and αi+1 is the recombination coefficient.

Coronal equilibrium is widely used to interpret X-ray and extreme ultraviolet spectra of the solar and stellar coronae. It underpins calculations of line emissivities, continua, and the differential emission measure, allowing diagnostics of the coronal temperature distribution and elemental abundances. Limitations arise when plasmas evolve rapidly, such as during solar flares, when non-equilibrium ionization (NEI) effects become important. External radiation fields or higher densities can also modify the balance.

See also: coronal heating, ionization balance, collisional ionization, radiative recombination, non-equilibrium ionization.