  Comparison of appropriate theoretical derived line ratios with observational
data can yield estimates of a plasma's physical parameters, such as electron
density or temperature. The usual practice in the calculation of the line ratio
is the assumption of excitation by electrons/protons followed by radiative
decay. Furthermore, it is normal to use the so-called coronal approximation,
i.e. one only considers ionization and recombination to and from the ground
state. A more accurate treatment is to include the ionization/recombination to
and from meta-stable levels. Here, we apply this to two lines from adjacent
ionization stages; Mg IX 368A and Mg X 625A, which has been shown to be a very
useful temperature diagnostic. At densities typical of coronal hole conditions,
the difference between the electron temperature derived assuming the zero
density limit compared with the electron density dependent
ionization/recombination is small. This however is not the case for flares
where the electron density is orders of magnitude larger. The derived
temperature for the coronal hole at solar maximum is around 1.04 MK compared to
just below 0.82 MK at solar minimum.
