= Ionization equilibrium
<Ionization equilibrium> means that the rates populating and depopulating ionization stages balance under a specified physical model. For a neutral species photoionized at rate $\Gamma$ per atom and singly charged <ions> recombining with coefficient $\alpha$, a simple balance is $n_0\Gamma=n_en_+\alpha$.
This kinetic balance need not be <local thermodynamic equilibrium>. Collisional ionization and radiative recombination can balance in a thin <plasma> that does not obey a Saha population law. Moreover, a rapidly cooling or flowing <gas> may retain a nonequilibrium ionization state if its ionization or recombination times exceed the evolution time. A <temperature> alone is therefore insufficient to declare ionization <equilibrium>.
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