1 Introduction

The EUV portion of the electromagnetic spectrum extends from 100 Å to 1000 Å, whereas the wavelength range from 1000 Å to 2000 Å is commonly referred to as FUV portion in the literature. The solar spectrum in these ranges contains not only a large number of bright emission lines, mainly from the chromospheric and lower transition region plasmas, i.e., formed at electron temperatures $T_{\rm e} \le 2.5\times10^5$ K, but also many coronal lines formed at temperatures of one million kelvin and above. However, above 1216 Å, no allowed lines from the upper transition region have previously been observed. Only a few forbidden lines that originate at coronal temperatures in plages had been reported in limb spectra (e.g., Brueckner 1981). In comparison, the solar spectrum in the range from 660 Å to 1200 Å includes a large number of lines from the upper transition region and corona, in addition to lines from the chromosphere and lower transition region (Curdt et al. 1997).

In this paper we present a spectral atlas representative of various solar features observed with SUMER on SOHO. SUMER is a high-resolution telescope and spectrograph designed to obtain stigmatic slit images with spatial and spectral resolution elements of $\approx$1 $^{\prime\prime}$ and $\approx$40 mÅ (in first order) as well as high temporal resolution over the wavelength range from 465 Å to 1610 Å. The accessible range depends on which of the detectors is used. While detector ``A'' can in principle record spectra from 780 Å to 1610 Å in first order of diffraction, the range of detector ``B'' reaches from 660 Å to 1500 Å. The gradual lower wavelength limit (for lines observed in second order) results from the steep fall-off of the reflectivity of the silicon carbide optics below 500 Å. The Ne VII line at 465 Å represents the shortest wavelength identified so far with this instrument. The range from 660 Å to 805 Å can be covered in both orders. More than 1100 emission lines are available in the SUMER spectral range. These include resonance lines as well as previously unobserved faint intersystem lines, which can be detected by SUMER because of its more efficient low-noise detectors compared to previous instruments, as discussed by Feldman et al. (1997) and Curdt et al. (1997). Thus, the SUMER spectral atlas provides a rich source of new diagnostic tools for probing essential physical properties of the emitting plasma and studying electron densities, electron temperatures and elemental abundances throughout the solar atmosphere.

Earlier EUV/FUV high-resolution instruments made use of UV-sensitive photographic plates (or film) as detectors. Because of the fairly low efficiency of the photographic plates used, few (if any) spectra were obtained from regions that extended more than 20 $^{\prime\prime}$ above the limb. In comparison, SUMER has recorded line intensities extending out to 600 $^{\prime\prime}$ (Feldman et al. 1999). Off-limb features are, however, excluded here and will be covered by a separate communication.

Given the radiometric calibration of SUMER before and during the mission (Hollandt et al. 1996; Wilhelm et al. 1997a; Schühle et al. 1998, 2000), we are able to present the absolute spectral radiances of different solar features in the wavelength range from 660 Å to 1485 Å in the first order of diffraction using the detector ``B''. For the sake of completeness, we have added the spectral range from 1485 Å to 1609 Å taken from a ``A''-detector spectrum of the quiet Sun. The calibrated radiance spectra have been compared to well-calibrated full disk irradiance measurements and reasonable agreement is found. (Dammasch et al. 1999a). The absolute value of the radiation in the EUV/FUV region is important for studying not only the solar transition region and corona, but also the atmosphere of the Earth. This radiation is an important source of energy in the upper atmosphere of the Earth and changes the temperature, chemistry and dynamics in these layers.

In Sect. 1, a brief summary is given of the most important EUV/FUV instruments and previously published solar spectral catalogues and line lists. In Sect. 2 we describe the instrument and the observation, while Sect. 3 deals with data reduction and calibration. The actual atlas is presented in Sect. 4. A list of all observed lines is added as annex to this atlas.