Astron. Astrophys. 351, 701-706 (1999)

2. Observational data and selected spectral lines

The observations were made on April 29-30, 1979 with the McMath-Pierce facility and the 1m Fourier Transform Spectrometer (FTS) polarimeter of the National Solar Observatory at Kitt Peak. The data consist of Stokes I and V spectra with a spectral resolving power of -, a noise level of approximately in units of the continuum intensity, , and a spatial resolution of approximately . A detailed description of these data is given by Stenflo et al. (1984). Here we analyse four spectrograms, 2 each observed in the network (identified as FTS2 and FTS3) and in an active-region plage (FTS4, FTS5). Together FTS2 and FTS3, respectively FTS4 and FTS5 cover a wavelength range from 457.4 to 685.8 nm. The overlapping wavelength ranges between FTS2 and FTS3, respectively FTS4 and FTS5 were used to give the V profiles in both spectra referring to the same kind of solar region a common polarisation scale, corresponding to the same magnetic filling factor (see Solanki et al. 1986). The filling factor is the fraction of the surface area covered by magnetic field.

We have selected unblended lines of different elements within this wavelength range using the line list of Gurtovenko & Kostik (1989). After individually checking all the selected Stokes V profiles, some lines were excluded from the initial list due to the presence of small blends, or the weakness of the Stokes V signal, which make them susceptible to noise. The weak lines (with relative depth , where ) are most affected by noise. They have generally been dropped, although a few weak lines of O and C have been retained because these elements have no lines with in the observed wavelength range. On the other hand, the strong lines (, mÅ) are affected by saturation and NLTE effects, making them relatively unreliable abundance indicators. They have therefore also been dropped. Our final list consists of 93 spectral lines of 13 elements. The elements, their FIP and the number of lines analysed for each element are listed in Table 1. Unfortunately, only 2 elements, C and O, have a FIP eV, since the present data set was not observed with the aim of deriving abundances. Nevertheless, it is unique in the sense that no other Stokes V spectrum combining such low noise with such a wide wavelength range has been obtained before or since.

Table 1. Elemental abundances derived for the quiet solar photosphere and inside magnetic flux tubes.

© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999
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