Upper panels: magnesium ionization fractions N_stage/N_total in the Kurucz model (left) and the FCHHT-B model (right), with N_stage the summed populations per ionization stage, N_total the total of all stages. Solid: NLTE. Dashed: LTE. Lower panels: gradient comparisons. The y-axis units specify the logarithmic electron density N_e and total hydrogen density N_H in particles m^-3. The temperature curves show 20   log (T) − 57 to provide comparable gradients.