DKIST resolves sub-arcsec photospheric scattering polarization

¹ Istituto ricerche solari Aldo e Cele Daccò (IRSOL), Faculty of Informatics, Università della Svizzera italiana, CH-6605 Locarno, Switzerland
² Euler Institute, Faculty of Informatics, Università della Svizzera italiana, CH-6900 Lugano, Switzerland
³ Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife, Spain
⁴ Departamento de Astrofísica, Facultad de Física, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain
⁵ High Altitude Observatory, National Center for Atmospheric Research, P.O. Box 3000 Boulder, CO 80307-3000, USA
⁶ National Solar Observatory, Makawao, Hawaii, USA
⁷ Consejo Superior de Investigaciones Cientifícas, Spain

^⋆ Corresponding author: zeuner@irsol.ch

Received: 29 September 2025
Accepted: 19 October 2025

Abstract

Scattering polarization signals offer unique diagnostics of the physical conditions in the solar atmosphere, in particular magnetic fields via the Hanle effect. However, their spatial structure remains poorly constrained due to the difficulty of achieving a high spatial resolution and polarimetric sensitivity simultaneously. We present the first direct observation of sub-arcsecond structuring in the linear scattering polarization of the photospheric Sr I 4607 Å line near the solar disk center (μ = 0.74), obtained with the Visible Spectro-Polarimeter (ViSP) at the Daniel K. Inouye Solar Telescope (DKIST). The data achieve about ∼0^″_.2 resolution with 30 s integration and sufficient sensitivity to detect fine-scale patterns in the total linear polarization, which are evident in Sr I but absent in a nearby Fe I line that is simultaneously observed. Since this Fe I line is more Zeeman-sensitive than the Sr I 4607 Å, this disparity confirms that the signals in the Sr I 4607 Å line arise from scattering. These data provide the first spatially resolved two-dimensional maps of photospheric scattering polarization at sub-arcsecond scales, enabled by the capabilities of a 4-meter solar telescope.