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

Abstract

Scattering polarization signals in the Sr I 4607 Å 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 Å 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 $μ= 0.74$, 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.