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Astron. Astrophys. 357, 743-756 (2000)

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2. Observations and data reduction

The SUMER spectrometer on SOHO is able to make extreme- to far-ultraviolet spectral observations within the 465 to 1610 Å range, with a pixel size between 0.042 and 0.044 Å (in the grating first order) and a spectral resolving power between 17700 and 38300. Simultaneous windows can be selected within the 1024 spectral pixels (43 to 45 Å, in first order) of the detector. Several entrance slits can be selected (from 0.3 x 120 arcsec2 to 4 x 300 arcsec2) and the solar image can be scanned with a step size, which is any multiple of 0.38 arcsec (see Wilhelm et al., 1995, 1997 and Lemaire et al., 1997). The two datasets analyzed in the present study were observed on 1996 September 16 and 17 (SEP16 and SEP17 respectively). They consist of two large quiet Sun rasters at disk centre corresponding to different grating positions. Each raster is built by steps of 0.76 arcsec on the solar image using a one arcsecond slit width. Details are given in Table 1.


Table 1. Details of SUMER observations on Sept. 16-17, 1996 ([FORMULA] and [FORMULA] indicate the grating first and second order).

The SEP16 dataset is the spectrum of a solar quiet region [FORMULA] arcsec2 with four spectral windows selected in order to include some strong transition region lines (N v and O v) together with a coronal line (Mg x). Wavelength ranges and spectral lines are given in Table 1. During the 60 second exposure time the Sun rotated by [FORMULA] 0.17 arcsec, so the effective raster step was 0.59 arcsec. The total observing time was 6h 46m.

SEP17 dataset consists of the spectrum of a solar quiet region [FORMULA] arcsec2 in the 1384-1428 Å spectral range, with 105 s exposure time and 0.47 arcsec effective raster step. This dataset includes a strong Si iv line and an O iv multiplet which provides density diagnostics for the transition region at around [FORMULA] K. The total observing time was 3h 15m.

Data reduction was carried out using standard routines allowing flat-field and radiometric calibration. Calibrated data units are phot cm-2 s-1 sr[FORMULA]. Data have been corrected for distortions in the image formed on the detector.

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Online publication: June 5, 2000