Evidence for the Hanle effect in molecular lines

¹ Institute of Astronomy, ETH Zentrum, 8092 Zurich, Switzerland e-mail: fluri@astro.phys.ethz.ch
² Astronomy Division, PO Box 3000, 90014 University of Oulu, Finland

Corresponding author: S. V. Berdyugina, sveta@astro.phys.ethz.ch

Received: 6 October 2003
Accepted: 15 December 2003

Abstract

In many wavelength regions molecular lines dominate the second solar spectrum that results from coherent scattering. Scattering polarization is modified by magnetic fields via the Hanle effect. This allows us to explore the magnetic field regime with weak field strengths and mixed polarities, which is not seen with the Zeeman effect and thus contains complementary information. Molecular lines are particularly well suited to diagnose such turbulent fields because they exhibit a broad range of magnetic sensitivities within narrow spectral regions. Thus, it is possible to employ the technique of the differential Hanle effect, i.e. to obtain field strengths by observing polarization ratios in various lines. We have identified one R- and one P-triplet of C₂ at 5140 Å and 5141 Å, respectively, that satisfy all conditions to be used in the differential Hanle effect. Based on these lines we have developed a model that can diagnose turbulent magnetic fields using the Hanle effect. The tool is sensitive over a broad range of magnetic field strengths from a few Gauss up to several hundred Gauss. This tool has allowed us to find a significant Hanle depolarization of C₂ lines in quiet Sun observations, which corresponds to a magnetic field strength of  G.