Emission lines from the solar corona

Abstract

My graduate work with Prof. Mike Seaton started back in the early 1970s when he presented me with a paper by Aly, Evans and Orrall (1962) on the total eclipse of the Sun, 25 February 1952 (figure 1). The paper concerned observations of the visible spectrum of the solar corona, the coronal forbidden lines – red, yellow, green and a few others. Mike told me that although some preliminary work had been carried out on these observations, a proper analysis could not be carried out because the atomic data available were inadequate. The coronal forbidden lines had presented a tremendous challenge to astronomers. Although the green line, 5303 A, was first observed in 1869, it remained a mystery for many years. Indeed a new element, coronium, was invented to explain it. Eventually, in 1939/41, with careful laboratory work, Edlen (1942) and Grotian identified many of the visible coronal lines as originating from highly ionized iron ions. The green line is the ground transition of Fe xiv, the red line of Fe x and the yellow line of Ca xv. Mike had taken a great interest in the work of Edlen at Lund, but was concerned that the ionization balance calculations were not fully consistent with the coronal observations, for example with coronal temperatures taken from spectral line widths. This puzzle was resolved by Mike and Alan Burgess, with the realization that dielectronic recombination plays an important role in the coronal plasma. In his paper “The spectrum of the solar corona” (Seaton 1964), Mike reviewed the interpretation of coronal observations in X-rays, UV, visible and radio wavelengths and presented the new ionization equilibrium for iron calculated by Burgess allowing for dielectronic recombination.