Observational insights into Sr I 4607 \AA\ scattering polarization with DKIST/ViSP

TL;DR

This study uses DKIST/ViSP to observe the Sr I 4607 Å scattering polarization in the solar photosphere, revealing sub-arcsec structures at the limb and highlighting current observational challenges at disk center.

Contribution

First high-resolution spectropolarimetric observations of Sr I 4607 Å scattering polarization with DKIST, identifying current limits and potential for future solar magnetic field diagnostics.

Findings

Sub-arcsec structures in scattering polarization observed at the limb.

Current noise levels prevent clear detection at disk center.

Instrumental corrections enable reliable reference lines for polarization measurements.

Abstract

Scattering polarization signals in the Sr I 4607 \AA\ spectral line are among the strongest originating from the solar photosphere, offering a powerful diagnostic of tangled magnetic fields in the 3--300 G range via the Hanle effect. However, measuring them with sub-arcsec resolution remains a significant challenge. We analyze spatially resolved quiet-Sun observations of these signals performed with the Visible Spectropolarimeter (ViSP) at the Daniel K. Inouye Solar Telescope (DKIST) and identify its current observational limits. We present high-resolution, high-precision spectropolarimetric observations in a spectral window including the Sr I 4607 \AA\ line at various limb distances. We apply consistent instrumental corrections across all spectral lines, enabling the adjacent lines to serve as reliable references. At a limb distance of $\mu = 0.74$, the signal-to-noise ratio is low but sufficient in the total linear polarization map to directly reveal sub-arcsec structures in the Sr I line for the first time, which can be attributed to scattering polarization. Disk-center measurements are still dominated by noise related to the current limitations of the observational setup. By combining high spatio-temporal and spectral resolution with exceptional polarimetric precision, DKIST enables measurements of solar photospheric scattering polarization at fine scales. However, current signal-to-noise limitations still hinder direct detection of disk-center scattering polarization and must be addressed before further progress can be made.