A Discussion on solar studies with special reference to space observations - Abundance of iron in the photosphere

Abstract
About ten years ago astrophysics was haunted by a nightmare called non-local thermodynamic equilibrium (n.l.t.e.). Some astrophysicists, notably Pecker (1959) and R .N . Thomas, began to doubt whether the well-known formulae for thermal excitation and ionization could be applied with sufficient accuracy to, for example, the solar photosphere. If that contention had been true, it would have meant that all solar abundance determinations were affected by large errors. On the other hand, it would have been a hopeless enterprise to develop a purely statistical theory for the distribution of the atomic states, for example, of neutral iron under the influence of the complicated field of radiation in the photosphere together with all sorts of collisions. After preliminary studies (Unsold 1962) had made it probable that in reality deviations from local thermodynamic equilibrium (l.t.e) were quite negligible in the photosphere and the lower chromosphere (but probably not so in the higher chromosphere and certainly not in the corona), Holweger (1967) proceeded to construct an empirical model of the solar photosphere (and lower chromosphere) in l.t.e. Since it was known (Pagel 1959; Unsold 1962; Lambert & Pagel 1968) that the negative hydrogen ion in any case conformed to the laws of l.t.e., the continuous spectrum and its centre limb variation could be used for obtaining the temperature distribution T(r0) for about 0.05 < t0 < 2. Just extrapolating this temperature distribution towards smaller r 0—as many authors have done would lead to very serious errors. So T ( t0) out to t0 10-6 was obtained using the observed intensities within strong lines, where the line absorption coefficient is much larger than the continuous absorption coefficient.
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