Equivalent Widths and the Temperature of the Solar Reversing Layer

Allen's extensive determinations of equivalent widths of Fraunhofer lines provide important observational material for an analysis of the physical state of the solar atmos- phere. A comparison of the observed intensities of absorption lines, as read from an em- pirical curve of growth, with the theoretical strengths of lines in a transition array makes it possible to calculate the effective excitation temperature of the reversing layer. Tem- peratures of 4350° ± 200° and 41500 ± 50° are computed from the lines of Ti i and Fe I, respectively. A qualitative discussion of the errors inherent in the theoretical calculation of mul- tiplet strengths is given, and a method for calculating the reversing-layer temperature by means of the f-file sum rule is described. The application of this method to the lines of Ti i yields a temperature of 4400° ± ba0. Since the sum rule is independent of the coupling in an atom, and is therefore free of the assumptions involved in the calculation of multiplet strengths, the value 4400° is adopted, for purposes of discussion, as the mean excitation temperature of the solar reversing layer. If the opacity of the solar atmosphere varies with wave length, we should expect to find the numbers of atoms, as derived from equivalent widths, depending upon wave length as well as upon the temperature and excitation potential. The data for Fe in- dicate an opacity law almost independent of wave length. These results, however, are not definitive. Since the mean lower excitation potentials increase systematically with wave length, opacity and temperature effects are correlated. The data for Ti, where no systematic correlation exists, are not inconsistent with an opacity varying as X~, whereas theory predicts a law varying approximately as X3e-hc/XkT. An attempt is made to rec- oncile the observations and the theoretical values