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Abstract: Coronal holes CH emit significantly less at coronal temperatures thanquiet-Sun regions QS, but can hardly be distinguished in most chromosphericand lower transition region lines. A key quantity for the understanding of thisphenomenon is the magnetic field. We use data from SOHO-MDI to reconstruct themagnetic field in coronal holes and the quiet Sun with the help of a potentialmagnetic model. Starting from a regular grid on the solar surface we then tracefield lines, which provide the overall geometry of the 3D magnetic fieldstructure. We distinguish between open and closed field lines, with the closedfield lines being assumed to represent magnetic loops. We then try to computesome properties of coronal loops. The loops in the coronal holes CH are foundto be on average flatter than in the QS. High and long closed loops areextremely rare, whereas short and low-lying loops are almost as abundant incoronal holes as in the quiet Sun. When interpreted in the light of loopscaling laws this result suggests an explanation for the relatively strongchromospheric and transition region emission many low-lying, short loops, butthe weak coronal emission few high and long loops in coronal holes. In spiteof this contrast our calculations also suggest that a significant fraction ofthe cool emission in CHs comes from the open flux regions. Despite theseinsights provided by the magnetic field line statistics further work is neededto obtain a definite answer to the question if loop statistics explain thedifferences between coronal holes and the quiet Sun.

Author: T. Wiegelmann, S.K. Solanki



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