2 Earlier spectral atlases

Space astronomy started in 1946 in the United States when W. von Braun's A4 rocket engines (during the war named V-2 missile) became available to scientists for launching free-flying, high-altitude observatories. It was not surprising, under these circumstances, that a group from the US Naval Research Laboratory (NRL) became the first to observe the ultraviolet radiation of the Sun with a spectrograph mounted on the tail fin of a A4 rocket. Since then, much progress has been made in solar high-resolution spectroscopy from space (see Feldman et al. 1988). A large number of experiments have been launched to observe the EUV and FUV portions of the solar spectrum. High spectral and spatial resolutions were achieved by several instruments at wavelengths above 1150 Å. At shorter wavelengths, however, the spectral and spatial resolution were moderate and only strong lines could be observed and/or identified. Spectral catalogues and line lists covering the FUV and EUV portions of the spectrum have previously been published by Burton et al. (1967), Burton & Ridgeley (1970), Dupree & Reeves (1971), Gabriel et al. (1971), Huber et al. (1973), Malinovsky & Heroux (1973), Kjeldseth-Moe et al. (1976), Doschek et al. (1976), Feldman et al. (1976a, 1976b), Vernazza & Reeves (1978), Dere (1978), Feldman & Doschek (1978), Cohen et al. (1978), Cohen (1981), Sandlin et al. (1986), Brekke et al. (1991), Feldman & Doschek (1991), Brekke (1993), Thomas & Neupert (1994), Brooks et al. (1999).

In particular, two instruments made a significant impact on solar spectroscopy: The NRL/S082B EUV spectrograph on the Skylab ATM (Apollo Telescope Mount) and the NRL/High Resolution Telescope and Spectrograph (HRTS). The S082B instrument operated in the 970 Å to 3940 Å range, but due to the coating (Al + MgF₂) of the optical surfaces, the instrument was very inefficient below 1100 Å (Bartoe et al. 1977). A spectral atlas covering the 1175 Å to 1950 Å wavelength range based on the S082B spectrometer was published by Cohen (1981). The 2 $^{\prime\prime}$ $\times$ 60 $^{\prime\prime}$ slit defined the spatial resolution element. Line lists based on the S082B data include those of Doschek et al. (1976), Feldman et al. (1976a, 1976b), Feldman & Doschek (1978), and Cohen et al. (1978), the latter being the most comprehensive. A few very long exposures of bright solar features resulted in useful spectra at wavelengths as short as 970 Å. A line list compiled from these observations was published by Feldman & Doschek (1991).

During the Skylab mission, several calibration rocket (CALROC) flights were carried out as part of a programme for calibration of the S082B spectrograph. A FUV spectral atlas in the wavelength range from 1170 Å to 2100 Å was prepared by Kjeldseth-Moe et al. (1976). The CALROC atlas contains the absolute intensity of an averaged quiet region located 300 $^{\prime\prime}$ inside the solar limb. During the observations the slit was rastered across its linear extension, thus averaging over an area of about 60 $^{\prime\prime}$ $\times$ 60 $^{\prime\prime}$ on the solar disk. In addition the atlas contains a quiet region 50 $^{\prime\prime}$ inside the solar limb while an active region is presented for wavelengths above 1680 Å. During these observations, the full spatial resolution of the instrument was used. The spectral resolution of the data is $\approx$70 mÅ and the radiance measurements have estimated relative uncertainties of $\pm 25 \%$ (1 $\sigma$) (Kjeldseth-Moe et al. 1976). This atlas was used as a reference calibration source for other FUV spectra for many years.

HRTS, which operated at wavelengths longer than 1150 Å, was the first spectrometer to combine both high spectral resolution (50 mÅ) and good angular resolution (1 $^{\prime\prime}$) with extensive wavelength and spatial coverage (Bartoe & Brueckner 1975). It was launched several times on sounding rockets and was part of the 5 day Spacelab 2 mission. Spectral catalogues based on the HRTS material covering the wavelength range from 1190 Å to 1710 Å have been published by Brekke et al. (1991) and Brekke (1993). These atlases represented different solar features that intercepted the 950 $^{\prime\prime}$ long slit extending from disk centre to the limb. The features included quiet Sun, plages, a sunspot (including a lightbridge), and an explosive event. A very comprehensive line list based on the HRTS material (and to some extent on the Skylab data) was published by Sandlin et al. (1986). The HRTS instrument has provided very important information on the properties of the solar chromosphere and transition region. Excellent reviews of some of the HRTS results can be found in the book ``The Solar Transition Region'' (Mariska 1992), and in Cook & Brueckner (1979).

The LASP (Laboratory of Atmospheric and Space Physics) EUV spectrometer has been flown on sounding rockets (Hassler et al. 1991). This instrument has been the only solar instrument with an onboard wavelength calibration lamp to provide an absolute wavelength reference with high spectral resolution. During the first flight the wavelength range was 1520 Å to 1600 Å (760 Å to 800 Å in second order). During a later rocket flight on March 12, 1998 the spectrometer was tuned to cover the spectral ranges from 1200 Å to 1280 Å in the first order and 600 Å to 640 Å in the second order.

Before the SOHO mission, the wavelength region between 660 Å and 1175 Å remained poorly observed. Spectra covering the 300 Å to 2950 Å wavelength range with a spectral resolution of about 0.4 Å were obtained during a series of rocket flights in the late 1960s and early 1970s by Burton et al. (1967), Burton & Ridgeley (1970), and Gabriel et al. (1971). Due to low spectral resolution, line shapes could not be obtained. However, many intense solar lines were identified. Improved observations of the EUV solar spectrum in the range from 280 Å to 1340 Å were obtained with the Harvard College Observatory (HCO) spectroheliometer S055 on Skylab (Reeves et al. 1977) and smaller instruments flown earlier on Orbiting Solar Observatories OSO-4 and OSO-6 (cf. Huber et al. 1973).

The S082A on Skylab was a slitless objective-type grating spectrograph covering the wavelength range from 170 Å to 630 Å (Tousey et al. 1977). Each exposure included numerous images of the entire solar disk with a spatial resolution of 2 $^{\prime\prime}$ $\times$ 2 $^{\prime\prime}$. An atlas of EUV spectroheliograms from 170 Å to 625 Å has been made available by Feldman et al. (1987).

Spectral catalogues at shorter wavelengths are those of Vernazza & Reeves (1978) and Malinovsky & Heroux (1973). Based on observations with the S055 spectrometer on Skylab, Vernazza & Reeves (1978) presented a spectral atlas of different solar features in the wavelength range from 280 Å to 1350 Å with a spectral resolution of 1.6 Å. Until now this atlas represented the most complete set of quiet-Sun spectra in this wavelength range. But due to poor spectral resolution of this spectrometer, many lines reported in this atlas are incorrectly identified. More recently an active region EUV atlas in the range from 230 Å to 450 Å was obtained with the SERTS (Solar EUV Rocket Telescope and Spectrometer) rocket experiment (Thomas & Neupert 1994). Emission lines of molecular hydrogen, which are present in sunspot spectra, have been measured and/or predicted by Bartoe et al. (1979). Brooks et al. (1999) have presented a spectral line list in the ranges 308 Å to 381 Å and 513 Å to 633 Å based on measurements of the normal incidence channel (NIS) of the SOHO/CDS instrument (Coronal Diagnostic Spectrometer, Harrison et al. 1995). A calibrated solar EUV spectrum in the same wavelength range has also been presented by Brekke et al. (2000).