Electron volume density needed to obtain the magnesium ionization equilibrium as a function of the temperature. At a lower density, the magnesium is mostly ionized, while the magnesium is mostly in its atomic form at higher density. The ionization rate is calculated using the electron-impact ionization rate extrapolated from the rate of Voronov (1997) that is combined with the photo-ionization rate (solid line) or the photo-ionization rate only (dashed line).