Inference of magnetic fields in inhomogeneous prominences

¹ Max-Planck-Institut für Sonnersystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
e-mail: milic@mps.mpg.de
² Astronomical Observatory Belgrade, Volgina 7, 11060 Belgrade, Serbia
³ UMR 7293 J. L. Lagrange Laboratory, Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, Campus Valrose, 06108 Nice, France
e-mail: marianne.faurobert@oca.eu
⁴ Department of astronomy, Faculty of Mathematics, University of Belgrade, Studentski Trg 16, 11000 Belgrade, Serbia
e-mail: olga@matf.bg.ac.rs

Received: 18 May 2016
Accepted: 30 August 2016

Abstract

Context. Most of the quantitative information about the magnetic field vector in solar prominences comes from the analysis of the Hanle effect acting on lines formed by scattering. As these lines can be of non-negligible optical thickness, it is of interest to study the line formation process further.

Aims. We investigate the multidimensional effects on the interpretation of spectropolarimetric observations, particularly on the inference of the magnetic field vector. We do this by analyzing the differences between multidimensional models, which involve fully self-consistent radiative transfer computations in the presence of spatial inhomogeneities and velocity fields, and those which rely on simple one-dimensional geometry.

Methods. We study the formation of a prototype line in ad hoc inhomogeneous, isothermal 2D prominence models. We solve the NLTE polarized line formation problem in the presence of a large-scale oriented magnetic field. The resulting polarized line profiles are then interpreted (i.e. inverted) assuming a simple 1D slab model.

Results. We find that differences between input and the inferred magnetic field vector are non-negligible. Namely, we almost universally find that the inferred field is weaker and more horizontal than the input field.

Conclusions. Spatial inhomogeneities and radiative transfer have a strong effect on scattering line polarization in the optically thick lines. In real-life situations, ignoring these effects could lead to a serious misinterpretation of spectropolarimetric observations of chromospheric objects such as prominences.