The quiet Sun extreme ultraviolet spectrum observed in normal incidence by the SOHO coronal diagnostic spectrometer

The extreme ultraviolet quiet Sun spectrum, observed at normal incidence by the Coronal Diagnostic Spectrometer on the SOHO spacecraft, is presented. The spectrum covers the wavelength ranges 308–381 Å and 513–633 Å and is based on data recorded at various positions on the solar disk between October 1996 and February 1997. Datasets at twelve of these ‘positions’ were judged to be free from active regions and data faults and selected for detailed study. A constrained maximum likelihood spectral line fitting code was used to analyse the spectral features. In all over 200 spectrum lines have been measured and about 50% identified. The line identification process consisted of a number of steps. Firstly assignment of well known lines was made and used to obtain the primary wavelength calibration. Variations of wavelengths with position were used to assess the precision of calibration achievable. Then, an analysis method first used in studies with the CHASE experiment, was applied to the new observations. The behaviour of the intensities of lines from like ions over the twelve positions, called ‘position patterns’, were used to distinguish probable emitters of weaker lines and extend the identifications. Spectral line widths and expected multiplet intensities were examined to identify lines and probable blends. The product of the study is a table which includes all clearly observed emission lines, their measured wavelengths, widths and count rates. Adopted laboratory wavelengths, ion and transition designations are also presented for identified lines. The table has an estimate of the uncertainty of the count rates based on a statistical analysis of the variability of each line. A marked spectrum is also provided.