The SUMER spectral atlas of solar-disk features

Abstract

A far-ultraviolet and extreme-ultraviolet (FUV, EUV) spectral atlas of the Sun between 670 Å and 1609 Å in first order of diffraction has been derived from observations obtained with the SUMER (Solar Ultraviolet Measurements of Emitted Radiation) spectrograph on the spacecraft SOHO (Solar and Heliospheric Observatory) [1]. The atlas contains spectra of the average quiet Sun, a coronal hole and a sunspot on disk. Different physical parameters prevalent in the bright network (BN) and in the cell interior (CI)-contributing in a distinct manner to the average quiet-Sun emission-have their imprint on the BN/CI ratio, which is also shown for the entire spectral range. With a few exceptions, all major lines are given with their identifications and wavelengths. Lines that appear in second order are superimposed on the first order spectra, but below 500 Å the responsivity of the normal-incidence optical system is very low. The spectra include emissions from atoms and ions in the temperature range 6 103 K to 2 106 K, i.e., continua and mission lines emitted from the lower chromosphere to the corona. This spectral atlas, with its broad wavelength coverage, provides a rich source of new diagnostic tools for studying the physical parameters in the chromosphere, the transition region and the corona. In particular, the wavelength range below 1100 Å as observed by SUMER represents a significant improvement over the spectra produced in the past. In view of the manifold appearance and temporal variation of the solar atmosphere it is obvious that our atlas can only be a-hopefully typical-snapshot. The spectral radiances are determined with a relative uncertainty of 0.15 to 0.30 (1σ), and the wavelength scale is accurate to typically 10 mÅ, which is the level achievable with semi-automatic processing. The SUMER solar-disk spectral atlas will be published in the near future by Curdt et al. [2]. It includes profiles of the average quiet Sun, an equatorial coronal hole, and a sunspot. As an example we show in Fig. 1 the spectral range from 1300 Å to 1342 Å with the prominent O I and C II lines. Resolved emission lines are indicated by a mark, the measured wavelength in angstrom, and the identification, if available. Marks point to line lists available in the literature, where additional information about a specific line can be found [3–7]. New lines or identifications are indicated. Lines observed in first order and in second-order of diffraction are distinguished. Only the three least-significant digits of the wavelength values are given. If available, unidentified lines are characterized by the temperature classification defined in [3] (a: Te<3 105; b: Te≈3 105; c: Te≈4 105; d: 6 105<Te <9 105; e: Te≈1.4 106; f: Te≈1.8 106). The vertical axes are scaled to spectral radiance in units of mW sr−1 m−2 Å−1; on the left the radiometric calibration for first order lines is given, on the right for second order lines. Note, that second order lines are always superimposed on a first order background. We have taken care of the type of photocathode (bare or KBr) when applying the radiometric calibration to different sections of the spectrum. Also displayed in green is the BN/CI ratio in an attempt to characterize the quiet-Sun chromospheric network structure. A pre-print of the SUMER spectral atlas and a line list is available at http://www.linmpi.mpg.de/∼curdt.