Similarities and Differences between Coronal Holes and the Quiet Sun: Are Loop Statistics the Key?

Abstract

Abstract. Coronal holes (CH) emit significantly less at coronal temperatures than quiet Sun regions (QS), but can hardly be distinguished in most chromospheric and lower transition region lines. A key quantity for the understanding of this phenomenon is the magnetic field. We use data from SOHO/MDI to reconstruct the magnetic field in coronal holes and the quiet Sun with the help of a potential magnetic model. Starting from a regular grid on the solar surface we then trace field lines, which provide the overall geometry of the 3D magnetic field structure.We distinguish between open and closed field lines, with the closed field lines being assumed to represent magnetic loops. We then try to compute some properties of coronal loops. The loops in the CH are found to be on average flatter than in the QS. High and long closed loops are extremely rare, whereas short and low-lying loops are almost as abundant in coronal holes as in the quiet Sun. When interpreted in the light of loop scaling laws this result suggests an explanation for the relatively strong chromospheric and transition region emission (many low-lying, short loops), but the weak coronal emission (few high and long loops) in coronal holes. In spite of this contrast our calculations also suggest that a significant fraction of the cool emission in CHs comes from the open flux regions. Despite these insights provided by the magnetic field line statistics further work is needed to obtain a definite answer to the question if loop statistics explain the differences between coronal holes and the quiet Sun.

Statistical Study of Network Jets Observed in the Solar Transition Region: a Comparison Between Coronal Holes and Quiet-Sun Regions

“…In CH, no such features can be observed, suggesting that, at the layers of TR in a CH, there are basically only open field lines and almost no loops. This result is also consistent with the findings of Wiegelmann and Solanki (2004); Tian et al (2008b) that loops reside only at very low layers in a CH. The difference in magnetic morphology in higher layers is likely responsible for the different propagation of the network jets in the two regions, leading to higher speed and longer distance in CHs.…”

“…In coronal images, CHs appear dark in comparison to QS because they emit less in ultraviolet and X-rays and are maintained at a lower temperature than the surrounding QS region. The different magnetic structures of CH and QS at coronal heights are responsible for their different appearance in coronal lines (Wiegelmann and Solanki, 2004;Tian et al, 2008b). CHs are dominated by open magnetic field lines expanding super-radially in the heliosphere, whereas QS regions are dominated by closed magnetic loops of different sizes.…”

The magnetic field in the solar atmosphere

“…Nevertheless, it already became clear that the photospheric field in which CHs are rooted is dominated by fields of a single polarity, i.e., either positive or negative. In ECHs, for instance, the main polarity of the holes dominated in the range of 58 to 95 % and, on average, 77 ± 14 % (Wiegelmann and Solanki 2004). Equatorial quiet-Sun regions were approximately flux balanced in the range of 2 to 29 % and, on average, 9 ± 9 %.…”

“…To our knowledge, a first review on observations of CHs and the underlying solar magnetic field was presented by Harvey and Sheeley (1979). Somewhat larger CH-averaged values, rang-ing from 0.8 to 17 G, with an average value of nearly 8 G, were found by Wiegelmann and Solanki (2004). Conclusions on the magnetic structure of CHs were at that time mainly based on the photospheric LOS magnetic field observations (e.g., from KPNO).…”

Observations of the Sun at Vacuum-Ultraviolet Wavelengths from Space. Part II: Results and Interpretations

“…7 showing the coronal holes observed by EIT in Fe XII on the same day). This behaviour of the transition-region emission has been explained with MDI magnetograms and magnetic-field extrapolations as being caused by the difference in the loop geometry: whereas loops are present in both regions, the loops in coronal holes are generally lower than those in quiet regions, and the high loops are entirely lacking in coronal holes (Wiegelmann and Solanki 2004). Similar conclusions were reached in the paper by on the magnetic structure of the solar transition region as observed in various ultraviolet lines emitted at different temperatures.…”