Multi-Line Quiet Sun Spectro-Polarimetry at 5250 and 6302 \AA

TL;DR

This study compares the diagnostic effectiveness of Fe I spectral lines at 5250 and 6302 Å for quiet Sun magnetic fields, highlighting the importance of inversion algorithms and multi-line analysis for accurate magnetic property determination.

Contribution

It provides a comprehensive analysis of high-resolution spectro-polarimetric data, demonstrating the limitations of line-ratio techniques and advocating for combined spectral range analysis and alternative lines.

Findings

Line-ratio technique at 5250 Å is unreliable with thermal fluctuations.

Inversion algorithms improve magnetic diagnostics over simple line ratios.

Combined analysis of multiple spectral lines enhances magnetic property estimation.

Abstract

The reliability of quiet Sun magnetic field diagnostics based on the \ion{Fe}{1} lines at 6302 \AA has been questioned by recent work. We present here the results of a thorough study of high-resolution multi-line observations taken with the new spectro-polarimeter SPINOR, comprising the 5250 and 6302 \AA spectral domains. The observations were analyzed using several inversion algorithms, including Milne-Eddington, LTE with 1 and 2 components, and MISMA codes. We find that the line-ratio technique applied to the 5250 \AA lines is not sufficiently reliable to provide a direct magnetic diagnostic in the presence of thermal fluctuations and variable line broadening. In general, one needs to resort to inversion algorithms, ideally with realistic magneto-hydrodynamical constrains. When this is done, the 5250 \AA lines do not seem to provide any significant advantage over those at 6302 \AA . In fact, our results point towards a better performance with the latter (in the presence of turbulent line broadening). In any case, for very weak flux concentrations, neither spectral region alone provides sufficient constraints to fully disentangle the intrinsic field strengths. Instead, we advocate for a combined analysis of both spectral ranges, which yields a better determination of the quiet Sun magnetic properties. Finally, we propose the use of two other \ion{Fe}{1} lines (at 4122 and 9000 \AA) with identical line opacities that seem to work much better than the others.