The validity of ionization equilibrium in highly ionized astrophysical plasmas

Ionization equilibrium is a useful assumption which allows temperatures and other plasma properties to be deduced from spectral observations of ionization stage densities. Inherent to this assumption is the premise that the ion stage populations are determined solely by atomic processes which are local functions of the plasma temperature and electron density. However, if the time scale of plasma flow through a temperature gradient is less than the characteristic time scale for the slowest atomic process, deviations from the ionization stage densities expected for ionization equilibrium will occur which could introduce serious errors into subsequent analyses. In this paper the coupled set of simplified mass conservation equations has been solved under conditions of steady flow through a range of temperature gradients. The results are presented in the form of detailed graphs showing the velocity: as function of ion species, temperature, temperature gradient, and electron density: at which the assumption of ionization equilibrium becomes questionable. In view of the flow speeds of kilometers per second which have been reported for both solar and stellar atmospheres, we conclude that the validity of ionization equilibrium must not be taken for granted. The elements and ion stages which are considered here are carbon (II--VII), nitrogen (III--VIII), oxygen (I--IX), neon( I--XI), magnesium (II--XII), silicon (I--XIV), sulfur (I--X), and iron (VI--VII). While some ions may reflect conditions appropriate to ionization equilibrium, for other ions ionization equilibrium may be a very poor assumption indeed.