Fraction of magnesium and iron in all ionizing stages that are in the Mg II 3p or Fe II (⁵D)4p states. The solid line is for magnesium and the dotted is for iron. The partition functions for the magnesium ionization stages are approximated with statistical weights for the corresponding ground levels. This is a good approximation at temperatures below 10⁴ K because there is now fine structure in the ground term of the three first ionization stages. For iron there are many levels of low excitation energy and the partition functions used here are from Halenka & Madej (2002). Below 10⁴ K the partition functions are almost constant through different lowering of the ionization energies. In this graph a lowering of the ionization energy of 0.05 eV and an electron density of 10⁹ cm^-3 is adopted.