A by-product of the ionization and excitation balance is the emissivity and opacity of the gas, which correspond to the net heating and cooling rates. Figure 6 shows the heating and cooling rates as a function of temperature and ionization parameter for the various elements. Heating rates are shown as solid curves, cooling rates as dashed curves. Rates assume solar abundances ([Grevesse, Noels and Sauval 1996]), and are given in units of erg s cm per H nucleus. Different curves correspond to ionization parameters log()=0,1,2,3,4 for an power law ionizing spectrum Fewer curves appear in some panels owing to pile up at low ionization parameters for elements such as carbon, while for H and He, the log curves fall below the range plotted. These are calculated in the limit of low gas density, n=1 cm.
A coronal plasma cools more efficiently, in general, than a photoionized plamsa since the ionization state is lower at a given temperature. Figure 7 shows the cooling rate as a function of temperature for such a plasma. Comparison of these rates with the results of Figure 6 shows similarity with the cooling rate at the lowest ionization parameter plotted there (log=0), although the coronal rates are generally larger at low temperatures. This is a reflection of the fact that at log=0 there is significant photoionization of the neutral and near-neutral species.