Detection of spatially structured scattering polarization of Sr I $4607.3$ \AA \ with the Fast Solar Polarimeter

TL;DR

This study measures and analyzes the scattering polarization of the Sr I 4607.3 Å line at near disk center using the Fast Solar Polarimeter, revealing spatial fluctuations and correlations with solar surface features.

Contribution

First high-resolution measurement of Sr I scattering polarization at μ=0.6, providing statistical analysis of spatial fluctuations and their relation to solar surface structures.

Findings

Polarization variation matches previous center-to-limb measurements.

Linear polarization correlates inversely with continuum intensity in intergranular lanes.

Patch size of polarization signals estimated at 0.5-1 arcseconds.

Abstract

Scattering polarization in the Sr I 4607.3 \AA \ line observed with high resolution is an important diagnostic of the Sun's atmosphere and magnetism at small spatial scales. At present, spatially resolved observations of this diagnostic are rare and have not been reported as close to the disk center as for $\mu=0.6$. Our aim is to measure the scattering polarization in the Sr I line at $\mu=0.6$ and to identify the spatial fluctuations with a statistical approach. Using the Fast Solar Polarimeter (FSP) mounted on the TESOS filtergraph at the German Vacuum Tower Telescope (VTT) in Tenerife, Spain, we measured both the spatially resolved full Stokes parameters of the Sr I line at $\mu=0.6$ and the center-to-limb variation of the spatially averaged Stokes parameters. We find that the center-to-limb variation of the scattering polarization in the Sr I line measured with FSP is consistent with previous measurements. A statistical analysis of Stokes $Q/I$ (i.e., the linear polarization component parallel to the solar limb), sampled with $0.16$" pixel$^{-1}$ in the line core of Sr I reveals that the signal strength is inversely correlated with the intensity in the continuum. We find stronger linear polarimetric signals corresponding to dark areas (intergranular lanes) in the Stokes $I$ continuum image. In contrast, independent measurements at $\mu=0.3$ show a positive correlation of $Q/I$ with respect to the continuum intensity. We estimate that the patch diameter responsible for the excess $Q/I$ signal is on the order of $0.5$"-$1$". The presented observations and the statistical analysis of $Q$/$I$ signals at $\mu=0.6$ complement reported scattering polarization observations as well as simulations.