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.