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

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Comparison of far-ultraviolet emission lines formed in coronal holes and the quiet Sun

K. Stucki 1, S.K. Solanki 2, U. Schühle 2, I. Rüedi 3, K. Wilhelm 2, J.O. Stenflo 1, A. Brkovi 1 and M.C.E. Huber 4

1 ETH-Zentrum, Institute of Astronomy, 8092 Zürich, Switzerland
2 Max-Planck-Institut für Aeronomie, 37191 Katlenburg-Lindau, Germany
3 PMOD/WRC, 7260 Davos Dorf, Switzerland
4 European Space Agency, Space Science Department, ESTEC, P.O. Box 299, 2200 AG Nordwijk, The Netherlands

Received 21 July 2000 / Accepted 21 September 2000

Abstract

We present an analysis of 26 far-ultraviolet emission lines belonging to 19 atoms and ions observed on both sides of the boundary of polar coronal holes as well as other quiet Sun areas along the limb. The observations were made with the SUMER instrument (Solar Ultraviolet Measurements of Emitted Radiation) onboard the Solar and Heliospheric Observatory (SOHO). We compare line intensities, shifts and widths in coronal holes with the corresponding values obtained in the quiet Sun. We find that with increasing formation temperature, spectral lines show on average an increasingly stronger blueshift in coronal holes relative to the quiet Sun at equal heliospheric angle, with the coolest lines in our sample (formation temperature [FORMULA] K) indicating a small relative redshift. With respect to the rest wavelength, however, only lines formed above [FORMULA] K show blueshifts in coronal holes, which is not very different from the quiet Sun. The width of the lines is generally larger (by a few kilometers per second) inside the coronal hole. Intensity measurements clearly show the presence of the coronal hole in Ne VIII lines as well as in Fe XII, and provide evidence for a slightly enhanced emission in polar coronal holes for lines formed below [FORMULA] K. This last result is, however, less certain than the rest due to relatively poor statistics. Intensity histograms also exhibit distinct differences between coronal hole and quiet-Sun data. For cooler chromospheric lines, such as Ni II, the coronal holes display a greater spread in intensities than the quiet Sun. Transition-region lines, e.g. O IV, do not reveal such differences, while Ne VIII shows characteristics of a coronal line with lower average intensity and lower intensity spread inside holes.

Key words: Sun: corona – Sun: solar wind – Sun: transition region – Sun: UV radiation

Send offprint requests to: K. Stucki

Correspondence to: kstucki@astro.phys.ethz.ch

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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