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

Autori
Stucki, Katja ; Solanki, Sami Khan ; Schühle, Udo ; Rüedi, Isabelle ; Wilhelm, Klaus ; Stenflo, Jan Olof ; Brković, Alen ; Huber, Martin Christoph Emanuele

Izvornik
Astronomy & Astrophysics (0004-6361) 363 (2000); 1145-1154

Vrsta, podvrsta i kategorija rada
Radovi u časopisima, članak, znanstveni

Ključne riječi
Sun: corona – Sun: solar wind – Sun: transition region – Sun: UV radiation

Sažetak
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 ≈ 10 000 K) indicating a small relative redshift. With respect to the rest wavelength, however, only lines formed above 500 000 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 100 000 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.

Izvorni jezik
Engleski

Znanstvena područja
Fizika

POVEZANOST RADA