Solar turbulent magnetic fields: Non-LTE modeling of the Hanle effect in the C₂ molecule

¹ Institute of Astronomy, ETH Zurich, 8093 Zurich, Switzerland
e-mail: kleint@astro.phys.ethz.ch
² Istituto Ricerche Solari Locarno (IRSOL), 6605 Locarno Monti, Switzerland
³ Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, Dorfstrasse 33, 7260 Davos Dorf, Switzerland
⁴ Kiepenheuer-Institut für Sonnenphysik, Schöneckstrasse 6, 79104 Freiburg, Germany

Received: 2 October 2010
Accepted: 3 October 2011

Abstract

Context. Scattering polarization measurements contain a wealth of information that needs a thorough interpretation. This often requires accounting for the non-local origin of photons with different frequencies and at different limb positions. Currently, modeling scattering polarization in several molecular C₂ lines simultaneously is only successful for lines with similar quantum numbers. More sophisticated models are needed to understand the dependence on quantum numbers and to reliably derive the strength of the turbulent magnetic fields using the differential Hanle effect.

Aims. We have developed a non-LTE analyzing technique for the C₂ lines to determine the strength of turbulent magnetic fields and have applied it to observations obtained during our synoptic program at the Istituto Ricerche SOlari Locarno (IRSOL).

Methods. The influence of magnetic fields on scattering polarization can be interpreted differentially, i.e., by comparing several spectral lines within one spectral region. Through the application of the differential Hanle effect and non-LTE 1D radiative transfer, we are able to infer a magnetic field strength from the photospheric C₂ lines around 5141 Å. Compared to previous models we include the effect of collisions and investigate their dependence on the total angular momentum number J.

Results. We carry out a detailed parameter study to investigate the influence of model parameters on the resulting scattering polarization. A good fit can now be obtained for spectral lines from different C₂ triplets. For the 78 measurements obtained during the solar minimum in 2007–2009 we infer a mean magnetic field strength of 7.41 G with a standard deviation of 0.76 G.