Evaluating the reliability of a simple method to map the magnetic field azimuth in the solar chromosphere

TL;DR

This study assesses the reliability of a simple method to map the magnetic field azimuth in the solar chromosphere using the Ca II 8542 Å line, considering observational noise and telescope size.

Contribution

It demonstrates that the azimuth mapping method is reliable despite inhomogeneities, with accuracy mainly limited by noise levels rather than telescope aperture.

Findings

The method yields accurate azimuth maps with low noise levels.

Large-aperture telescopes improve measurement reliability.

Intrinsic ambiguities exist but do not significantly impair results.

Abstract

The Zeeman effect is of limited utility for probing the magnetism of the quiet solar chromosphere. The Hanle effect in some spectral lines is sensitive to such magnetism, but the interpretation of the scattering polarization signals requires taking into account that the chromospheric plasma is highly inhomogeneous and dynamic (i.e., that the magnetic field is not the only cause of symmetry breaking). Here we investigate the reliability of a well-known formula for mapping the azimuth of chromospheric magnetic fields directly from the scattering polarization observed in the \ion{Ca}{2}~8542~\AA\, line, which is typically in the saturation regime of the Hanle effect. To this end, we use the Stokes profiles of the \ion{Ca}{2}~8542~\AA\, line computed with the PORTA radiative transfer code in a three-dimensional (3D) model of the solar chromosphere, degrading them to mimic spectropolarimetric observations for a range of telescope apertures and noise levels. The simulated observations are used to obtain the magnetic field azimuth at each point of the field of view, which we compare with the actual values within the 3D model. We show that, apart from intrinsic ambiguities, the method provides solid results. Their accuracy depends more on the noise level than on the telescope diameter. Large-aperture solar telescopes, like DKIST and EST, are needed to achieve the required noise-to-signal ratios using reasonable exposure times.