Quiet Sun Magnetic Field Measurements Based on Lines with Hyperfine Structure

TL;DR

This paper investigates the use of MnI lines with hyperfine structure for quiet Sun magnetic field measurements, revealing biases in previous methods and demonstrating the complex interplay of weak and strong fields in polarization signals.

Contribution

First synthesis of MnI lines in realistic quiet Sun models showing how hyperfine structure affects magnetic field measurements and biases in polarization signals.

Findings

MnI lines weaken with increasing magnetic field strength.

Circular polarization signals can be much smaller than expected in strong fields.

Weak and strong fields influence polarization differently, biasing measurements.

Abstract

The Zeeman pattern of MnI lines is sensitive to hyperfine structure (HFS) and, they respond to hG magnetic field strengths differently from the lines used in solar magnetometry. This peculiarity has been employed to measure magnetic field strengths in quiet Sun regions. However, the methods applied so far assume the magnetic field to be constant in the resolution element. The assumption is clearly insufficient to describe the complex quiet Sun magnetic fields, biasing the results of the measurements. We present the first syntheses of MnI lines in realistic quiet Sun model atmospheres. The syntheses show how the MnI lines weaken with increasing field strength. In particular, kG magnetic concentrations produce NnI 5538 circular polarization signals (Stokes V) which can be up to two orders of magnitude smaller than the weak magnetic field approximation prediction. Consequently, (1) the polarization emerging from an atmosphere having weak and strong fields is biased towards the weak fields, and (2) HFS features characteristic of weak fields show up even when the magnetic flux and energy are dominated by kG fields. For the HFS feature of MnI 5538 to disappear the filling factor of kG fields has to be larger than the filling factor of sub-kG fields. Stokes V depends on magnetic field inclination according to the simple consine law. Atmospheres with unresolved velocities produce asymmetric line profiles, which cannot be reproduced by simple one-component model atmospheres. The uncertainty of the HFS constants do not limit the use of MnI lines for magnetometry.