A discussion of theoretical ionization equilibrium calculations based on solar flare X-ray spectra

Several sets of ionization equilibrium calculations exist for use in interpreting X-ray and EUV spectra of astrophysical plasmas. In particular, the calculations of Jordan (1969, 1970), Jacobs et al. (1977, 1978) and Summers (1974) are well known. The temperatures of maximum fractional abundance calculated by Summers for the more highly ionized and heavier elements such as iron are about a factor of two higher than the temperatures calculated by Jordan and Jacobs et al. By the use of recently obtained X-ray spectra of solar flares, it is shown that the temperatures calculated by Summers (1974) for iron are incorrect. The temperatures calculated by Jordan or Jacobs et al. should be used until further improvements become available.