Classification of Iron Lines in the Spectrum of the Sun and Zeta in the Range 167 Å to 220 Å

Abstract

A NUMBER of teams^1–3 using rocket or satellite-borne spectrometers have made observations of a group of intense unidentified emission lines in the solar spectrum between 167 Å and 220 Å. The origin and classification of these lines have aroused considerable interest in a number of laboratories. The lines were first reported from laboratory sources by Fawcett et al.⁴, who produced them in the Zeta discharge at Harwell. There were strong indications that iron from the walls of the discharge vessel was responsible. Further evidence for iron was provided by Elton et al.⁵ and House, Deutsehmann and Sawyer⁶, who produced the same lines by adding iron to θ-pinch discharges. Fawcett and Gabriel⁷ at Culham showed that the lines could be produced in various iron sparks, and also produced similar systems of intense lines from all elements in the period from calcium to nickel. From the wave number variation in these elements, they were able to state that 3p–3d transitions were responsible. By studying the intensity variation with exciting energy they obtained a correlation with ions between FeIX and FeXIII. Alexander, Feldman and Fraenkel⁸ observed some of these lines in several sparks of different types, and assigned degrees of ionization as FeVII to FeX. Neupert⁹ has made an attempt to assign ionization states by correlating the intensities from the Sun with periods of solar activity, and proposed ions between FeX and FeXIV. Zirin¹⁰ in an analysis of solar data² has tentatively classified three lines in this spectral region as FeXIII lines.