The Temperature Dependence of Line Ratios of Helium-Like Ions

In a recent paper Blumenthal et a!. have suggested that the use of helium-like intercombination to forbidden line ratios to measure electron densities is complicated by an important electron- temperature dependence of the ratios. It is shown here that their theory uses an incorrect treatment of dielectronic recombination and that the temperature dependence is in fact smaller. It is concluded that significant observations of ratios which vary by more than I 10 percent can safely be attributed to electron density effects. Recent observations, in particular those of Acton et a!. of O vii, show variations of only 15 percent. Although Acton et a!. consider that their data support the temperature dependence predicted by Blumenthal et a!., they can equally well agree with a much lower temperature dependence. However, the accuracy of the observations to date is not sufficient to show a definite dependence on temperature. Subject headings: atomic processes - atmospheres, solar