Astron. Astrophys. 363, 1145-1154 (2000)

4. Conclusions

We have analysed the line intensity, shift, and width of 26 spectral lines observed by SUMER in polar coronal holes and compared these with the corresponding parameters determined in quiet-Sun regions.

We find evidence for the presence of a coronal hole through strongly reduced intensity in spectral lines formed above K. The distribution of intensities shows more bright network inside coronal holes for chromospheric lines, but this effect disappears in transition-region lines. The paucity of the statistics means that it is important to study such distributions with considerably larger data sets, as has recently been done by Pauluhn et al. (2000) for the quiet Sun. The line widths are found to be larger inside coronal holes. The excess widths correspond approximately to excess turbulence velocities of 10-15 in coronal holes.

A blueshift in the coronal hole relative to the quiet Sun is shown by all the lines with formation temperatures above  K, and it increases with temperature. The interesting possibility that the blueshifts observed by Hassler et al. (1999), Wilhelm et al. (1998b, 2000) and Dammasch et al. (1999) reveal the solar wind acceleration in the polar coronal holes needs further examination. We have extended the work of these authors in that we consider a larger number of spectral lines. We have also focussed on the wavelength shifts in quiet sun and coronal holes separately (determined under the asumption that the quiet-Sun shifts derived by Teriaca et al. are correct). These reveal that the shifts between lines in coronal holes and the quiet Sun are small compared to the shifts exhibited by the lines in the quiet Sun. This indicates some caution in attributing the relative blueshifts observed in coronal holes to the fast solar wind.

It would be of great interest to have better access to the properties of the coronal gas in a hole, in order to try to follow plasma movements in hotter gas. The scarcity of truly coronal lines in the SUMER spectral range, and in particular in the analysed data, is a hinderance. The CDS instrument onboard SOHO may give further insight by providing the possibility of analysing more lines with higher formation temperatures as well as with better statistics, although with lower spatial and spectral resolution. Such work is currently in progress.

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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