A study of opacity in SOHO-SUMER and SOHO-CDS spectral observations I. Opacity deduction at the limb

A study is presented of the optical thickness of spectral lines of carbon, nitrogen and oxygen ions in the quiet sun. The observations consist of cross limb scans by the SUMER and CDS spectrometers on the SOHO spacecraft. A maximum likelihood spectral line fitting code has been adapted to analyse the multiplet profiles and to provide an assessment of errors in the count rates, especially of close lying components. Branching multiplet component ratios are presented as a function of position across the limb and contrasted with theoretical ratios in the optically thin case. The emergent fluxes are analysed in an escape probability model to deduce the optical thicknesses in the various spectral lines. Different specifications of the escape probability are examined. These are used to compare the observations with a geometric model of the emitting layer thickness across the limb and the thinning of the emitting layer above the limb. Classification of the deviations of quiet sun spectral line intensities from the optically thin case is given to assist in the critical selection of lines for differential emission measure analysis. This is linked to a general purpose code for the calculation of the influence of the line radiation fields on the local excited state population structure of the selected ions so that the fluxes in any spectral lines can be predicted. The Atomic Data and Analysis Structure (ADAS) was used for the atomic calculations and data of the paper.