Astron. Astrophys. 357, 697-715 (2000)

1. Introduction

The effects of opacity in solar spectral lines have been studied on a number of occasions (Jordan 1967; Doschek et al. 1976; Doyle & McWhirter 1980). Jordan established the technique of using branching ratios of lines arising from a common upper level to extract information on opacities from spectral observations. Doyle & McWhirter subsequently developed this same technique to study opacity at the solar limb and their work included a simple model of predicted line ratios from the region on-disk up to the limb. Escape probability and escape factor expressions have been developed by many authors (eg. Holstein 1947; McWhirter 1965; Irons 1979). More recently Kastner & Kastner (1990), Kastner & Bhatia (1989) and Kastner & Bhatia (1992) have developed extensively escape probability expressions and have used them for examining emergent intensities and optically thick population structures, both from a predictive point of view.

The present paper seeks to use and develop the quantitative approach of Doyle & McWhirter in an analysis of spectral data from the SOHO spacecraft spectrometers CDS and SUMER. There are three objectives in this new analysis. Firstly, the range of multiplets available in this study enables us to present some systematics of the incidence of opacity along iso-electronic sequences and for different types of line transition. Secondly, we extend the Doyle & McWhirter analysis to the case of lines with optical thicknesses significantly greater than unity on the solar disk. The Doyle & McWhirter specification of the escape probability is imprecise in these circumstances and can be improved by layer averaging. Doyle & McWhirter used a geometric model of the emitting layer thickness variation at the limb to describe the line ratio observations. The present data quality allows also modelling of the line ratios from the thinning atmosphere above the limb for which we construct three alternative models. Thirdly, we seek to be able to judge the suitability of lines observed by CDS and SUMER intended for use in differential emission measure analysis. Such analysis is subject to criticism on grounds of uncertainties of theoretical atomic physics, static stratified atmosphere assumptions and line selection and calibration. The present study allows the prediction of those lines which should be rejected (or intensity adjusted) because they are not optically thin.

We take a special interest in assessing how atmospheric substructure affects our opacity analysis and so consider the effects of local brightenings on-disk, wavelength shifts on crossing the limb, spicule like structures off-limb and the effects of instrumentally scattered light.

© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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