Hidden magnetic fields of the quiet Sun derived from Hanle depolarization of lines of the "second solar spectrum" at the limb from Pic du Midi observations

TL;DR

This study analyzes high-resolution polarization spectra of the quiet Sun's limb to infer hidden magnetic fields using Hanle depolarization, revealing a turbulent magnetic field of 13-25 Gauss with an altitude-dependent gradient.

Contribution

It provides new measurements of the turbulent magnetic field in the quiet Sun using Hanle depolarization of the second solar spectrum from Pic du Midi observations.

Findings

Turbulent magnetic fields of 13-25 Gauss were detected.

Magnetic field strength often decreases towards the solar limb.

Spectral data and analysis methods are now publicly available.

Abstract

This paper is based on a dataset of many strongly polarized solar lines belonging to the ''second solar spectrum'', i.e. the spectrum near the limb in linear scattering polarization. The observations were done at the Pic du Midi Turret Dome in 2006. The solar spectra were recorded at high spectral resolution (R = 400000) with the spectrograph slit orthogonal to the solar limb, so that $\mu$ = cos$\theta$ continuously varied from 0 .0 to 0.45. The crystal liquid polarimeter delivered the linear polarization rate (Q/I). Strong lines such as CaII 3934 {\AA}, CaI 4227 {\AA}, SrI 4607 {\AA}, SrII 4078 {\AA}, BaII 4554 {\AA} were studied. We measured the Hanle depolarization with the help of models predicting the polarization envelope with no magnetic field and we got values in the range 13-25 Gauss for the unresolved turbulent magnetic field, and we found that it often decreases towards the limb, revealing an altitude gradient. This present analysis was not yet published and spectra shown here become freely available to the research community.