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Astron. Astrophys. 340, L15-L17 (1998)

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2. Observational data and reduction

SUMER is designed to study the solar chromosphere, transition region and corona, and has a resolving power of [FORMULA] 17700-38300 (first order) at 800-1610 Å, and an angular resolution of 1 arcsecond (see Wilhelm et al. 1995). This is an ideal instrument for measuring detailed spectroscopic diagnostic line ratios in various solar features. The spectral regions selected for our observations were from 1213 to 1215 Å, and from 1217.5 to 1219.5 Å, for the forbidden and intercombination lines respectively. These regions lie on the wings of the Lyman-[FORMULA] feature (1215.6 Å), and the Lyman-[FORMULA] feature was placed on one of SUMER's two 1:10 attenuators, located near the detector edge.

The observations were carried out on September 2nd 1996 on a quiet region. The 1[FORMULA]300 arcsecond slit (orientated North-South), was centred 318 arcseconds east of Sun centre, 154 arcseconds north of Sun centre. A raster of 45[FORMULA]300 arcseconds was performed, comprising sixty separate 30 second exposures with a raster increment of 0.753 arcseconds east, in each spectral region. The O V forbidden line can be seen in each of the 60 spectra at the expected wavelength.

The nonuniform detector sensitivity creates a pattern of hexagon shapes and small circular bumps in the data, which were removed by flat-fielding. The data were also radiometrically calibrated, and destretched (a process that corrects for both wavelength and spatial distortions on the detector), using standard SUMER software. A temporal average of all 60 spectra in each wavelength region is shown in Fig. 1.

[FIGURE] Fig. 1. The temporally averaged spectra of the forbidden and intercombination line regions, totalling 1800 seconds each. The lines are centred in the middle of each plot, and the instrumental count rates have been radiometrically calibrated. No separation of O V lines and the Lyman-[FORMULA] wings has been performed.

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© European Southern Observatory (ESO) 1998

Online publication: November 3, 1998