1 Introduction

The Coronal Diagnostic Spectrometer (CDS) on SOHO consists of two instruments: a Grazing Incidence Spectrometer (GIS) producing astigmatic spectra; and a Normal Incidence Spectrometer (NIS) which can provide images of the solar field of view, dispersed over two wavelength ranges. Together, GIS and NIS cover most of the extreme ultraviolet wavelength range from 150 Å to 785 Å (Harrison et al. 1995). The GIS instrument has four detectors, with spectral ranges 151 Å - 221 Å, 256 Å - 341 Å, 393 Å - 492 Å and 659 Å - 785 Å, while NIS covers the ranges 308 Å - 379 Å and 513 Å - 633 Å.

This spectral region is extremely rich in emission lines from a large number of highly ionized ions of the most abundant elements, providing a unique diagnostic tool for investigating the temperature, density and chemical composition of the solar transition region and corona. Moreover it provides a laboratory for testing atomic physics models and theoretical calculations of collision rates and transition probabilities for ions formed at chromospheric to coronal temperatures.

Intensity calibration is a fundamental requirement for many scientific uses of the spectrometer, but obtaining such information is not an easy undertaking in the extreme ultraviolet spectral range. A pre-flight calibration was performed (Bromage et al. 1996) using a limited number of emission lines, providing an initial measure of the mean sensitivity of each detector and a basis for absolute calibration of the instrument. Exposure to air between calibration and launch, as well as exposure to solar radiation in flight, will have modified the efficiency of the detectors in at least some regions. The large number of emission lines that are observed in the solar spectrum, coupled to the large amount of atomic data now available (CHIANTI - Dere et al. 1997, The Arcetri Spectral Code - Landi & Landini 1998a, ADAS - Summers et al. 1996) and a new temperature and density diagnostic technique (Landi & Landini 1997), allow a more detailed determination of the current (in-flight) relative intensity calibration.

This new diagnostic technique has been applied to GIS observations of a solar active region (AR) and of a quiet Sun region (QS), in order to derive correction factors which may be applied to the pre-flight intensity calibration of the instrument. This method has already been successfully applied to the CDS NIS spectrometer (Landi et al. 1997).

Since the aim of this work is to assess the calibration of the GIS instrument, only a brief description of the emission lines used for this purpose is provided, leaving a more complete description of a full line list and of the diagnostic capabilities to a further paper. Together with the NIS quiet Sun spectral atlas (Brooks et al. 1998a), these line lists represents a spectral atlas for solar EUV line emission under both quiet Sun and active region conditions.