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Astron. Astrophys. 346, 285-294 (1999)

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4. Observations and data overview

In September 1996, a SUMER observational sequence was executed to determine the Doppler shifts of the Ne VIII ([FORMULA]770) and the C IV lines. The sequence was run twice. First on September 21, 1996 from 00:16 to 07:31 UT near the north polar coronal hole; then on September 22, 1996 from 00:40 to 08:14 UT in a mid-latitude region on the solar disk. In both cases, a wavelength of 1541.5 Å was positioned on reference pixel 531 of detector A, and half of the detector array (512 spectral [FORMULA] 360 spatial pixels) around that position was transmitted to the ground. This provides us with a spectral window of [FORMULA] 20 Å on the KBr photocathode as shown in Fig. 2 for a portion of the second run. A slit with angular dimensions of 1" [FORMULA] 300" was used, spectrally resolved images of the slit were taken with an exposure time of 150 s each, and the field of view was rastered with a step size of 8 elementary steps corresponding to 3:0001 in east-west direction. On the first day, exposures were performed at 173 raster positions between 121" east and 397" west, with the centre of the slit pointing to 780" north (from 630" to 930"). On the second day, 181 exposures were taken between 476" east and 66" west centred at 411" north (from 261" to 561"). No onboard flat-field correction was used. The fields of view are shown in Fig. 3 inserted in an EIT image (Extreme-ultraviolet Imaging Telescope, Delaboudinière et al. 1995) taken between the two SUMER rasters. The spectral window contains the strong lines of Si II and Ne VIII as well as the C IV doublet. These lines and the adjacent continuum radiation have been used to visualize in Figs. 4 and 5 the properties of the solar atmosphere at different temperature levels, namely at 14 000 K in the Si II ([FORMULA]1533) line, at 100 000 K in the C IV ([FORMULA]1548) line, and at 630 000 K in the Ne VIII ([FORMULA]770) line. The carbon line is a typical transition region line, whereas Si II and Ne VIII show the situations at the lower and higher temperature levels. The Li-like Ne[FORMULA] ion is particularly well suited for observations over a wide temperature range, because of the long tail towards high temperatures of its contribution function, [FORMULA], (cf., Doschek et al. 1998; Laming et al. 1998).

[FIGURE] Fig. 2. The SUMER spectrum in the wavelength range from 1531.6 Å to 1552.2 Å in first order (765.8 Å to 776.1 Å in second order) observed on September 22, 1996 around 06:00 UT. The spectrum of the quiet Sun has been averaged over 83 raster steps (the exposures in the west of the scan) and shows the bright lines of Si II , Ne VIII (in second order) and C IV , in addition to many weak Si I lines, some C I lines plus the Fe II ([FORMULA]1550.260) and N II ([FORMULA]775.965) (in second order) lines. Their spectral positions are marked by dots (except for the 144 Si I lines). The radiometric calibration has been performed with the standard SUMER software and is valid for the first-order lines. The radiances of the second-order lines have to be reduced by a factor of 3.38 to match this calibration. The horizontal bars indicate the ranges which will later be used for an absolute wavelength calibration of Ne VIII ([FORMULA]770).

[FIGURE] Fig. 3. The Sun seen in the wavelength band near 171 Å (Fe IX , Fe X ) by EIT/SOHO on September 21, 1996 at 19:00 UT (by courtesy of the EIT consortium). The raster scans of SUMER are shown as rectangles near the limb on September 21, 1996 and at mid-latitudes on September 22, 1996. The areas indicated in the west of 83 raster steps each have been used to deduce the Ne VIII wavelengths.

[FIGURE] Fig. 4. Diagrams showing the September 21, 1996 results. The flat-field corrections and detector destretching routines outlined in Sect. 5.1 have been applied to these data. The first row gives line intensities, the second continuum intensities, the third line shifts and the fourth line widths. The different lines are arranged in three columns. Note the similarity of the continuum maps. Line radiances in units of photon s-1m-2sr-1 are between [FORMULA] and [FORMULA] (Si II ), [FORMULA] and [FORMULA] (C IV at 1548 Å), and [FORMULA] and [FORMULA] (Ne VIII ). Continuum intensities are between [FORMULA] and [FORMULA] photon s- 1m-2sr-1Å-1. Line shifts are between [FORMULA] 30 km s-1. Line widths are between 181 mÅ and 544 mÅ in FWHM of the Gauss fits (not corrected for instrumental effects) or 19 km s-1 and 63 km s-1, respectively, in corrected Doppler width.

[FIGURE] Fig. 5. Display of the data of September 22, 1996 for a quiet Sun region using the same arrangements and calibration factors as in Fig. 4.

Not only the line characteristics (intensity, Doppler shift and line width) are of interest, but also the continuum radiation. It was estimated by determining "continuum" pixels in a spectral window around each line. The background correction was performed by subtracting the continuum value from pixels containing "line" information, and their remaining values were integrated to calculate the line intensity. Further details of the data analysis methods will be discussed in the next section.

An exhaustive analysis of the diagrams shown in Figs. 4 and 5 is beyond the scope of this contribution, but a few salient characteristics should be mentioned: (1) The chromospheric network can easily be recognized in the intensity maps of both the coronal hole and the quiet Sun regions for the Si II and C IV ([FORMULA]1548) lines and the continua. (2) The network appears to be very diffuse for the quiet Sun in Ne VIII and disappears in the coronal hole. (3) The velocity maps confirm the intensity results. There is no significant difference between coronal hole and quiet Sun regions for C IV and Si II , but the Ne VIII line is shifted much more towards blue in the hole, however, less so in areas bright in this line. (4) The sizes of the upwards and downwards moving structures increase with increasing temperatures. (5) The line widths are, in general, positively correlated with the line intensities for all lines.

These observations also form the basis of studies by Hassler et al. (1999) and Dammasch et al. (1999), which concentrate on the outflow speeds in quiet Sun areas as well as in coronal holes deduced from the Ne VIII and Si II lines and on a statistical analysis of the lines. In what follows, we will focus our attention on the wavelength determinations for the Ne VIII ([FORMULA]770) line and defer a detailed investigation of the other lines.

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© European Southern Observatory (ESO) 1999

Online publication: May 6, 1999