| Presenter: | Andreas Lagg |
| Affiliation: | MPI für Sonnensystemforschung |
| Title: | Measurements of canopy fields at chromospheric heights |
| Authors: | A. Lagg, S.K. Solanki, J. Woch, A. Gandorfer |
| Form: | talk |
| Abstract: | The expansion of magnetic flux tubes or sheets with increasing height is a direct consequence of the decrease in gas pressure. Gabriel (1976) first suggested that the magnetic field forms a nearly horizontal canopy and Giovanelli (1980) presented first evidence for the presence of low-lying canopies. An explanation for low-lying canopies was provided by Solanki and Steiner (1990) in the context of an inhomogeneous chromospheric thermal structure. In these models, magnetic flux escaping from intergranular lanes expands rapidly with increasing height, leading to the formation of horizontal field lines spanning over supergranular cells. Numerous measurements give strong evidence for canopy like fields (e.g.\ the Hanle depolarization in Sr~{\sc{ii}} by Bianda et al.\ (1998, 1999), Zeeman and Hanle diagnostics in Na D$_2$ by Stenflo et al.\ (2002), or the acoustic mapping of the plasma-$\beta=1$ surface by Finsterle et al.\ (2004). However, there are also indications that the concept of a magnetic canopy is fundamentally wrong. Schrijver and Title (2003) computed that the presence of unresolved, relatively strong inter-network field concentrations of the order of a few Mx\,cm$^{-2}$ destroy the classical, wineglass shaped canopy. Here we present data obtained with the Tenerife Infrared Polarimeter 2 (TIP-2) mounted behind the German Vacuum Tower Telescope (VTT) on Tenerife. Highly sensitive polarimetric measurements in the He~{\sc{i}} 10830~{\AA} triplet are analyzed using sophisticated inversion techniques involving the Zeeman, Hanle, and Paschen-Back effects. We show that the magnetic field in the upper chromosphere, as sampled by the He~{\sc{i}} line, lies horizontally over a supergranular cell, indicating that the majority of field lines at these heights are rooted in the network. |
| Session: | 3. Nature of solar magnetic fields as revealed by polarimetry |
| Presentation date: | Tuesday 18th September |
| Presentation time: | 11:15:00 |