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

TL;DR

This study investigates the magnetic field structures in coronal holes and quiet Sun regions, revealing differences in loop geometries that may explain their distinct emission characteristics, with implications for understanding solar atmospheric phenomena.

Contribution

It provides a detailed analysis of magnetic loop geometries in coronal holes versus quiet Sun, highlighting the role of loop statistics in solar emission differences.

Findings

Coronal hole loops are flatter and shorter than quiet Sun loops.

High, long loops are rare in coronal holes.

A significant fraction of cool emission in coronal holes originates from open flux regions.

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 coronal holes (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.