Detection of spatially structured scattering polarization of Sr i 4607.3 Å with the Fast Solar Polarimeter

Abstract

Context. Scattering polarization in the Sr I 4607.3 Å line observed with high resolution is an important diagnostic of the Sun's atmosphere and magnetism at small spatial scales. Investigating the scattering polarization altered by the Hanle effect is key to constraining the role of small-scale magnetic activity in solar atmospheric activity and energy balance. At present, spatially resolved observations of this diagnostic are rare and have not been reported as close to the disk center as for μ = 0.6. Aims. Our aim is to measure the scattering polarization in the Sr I line at μ = 0.6 and to identify the spatial fluctuations with a statistical approach. Methods. 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 μ = 0.6 and the center-to-limb variation of the spatially averaged Stokes parameters. Results. 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 in the Stokes I continuum image (intergranular lanes). In contrast, independent measurements at μ = 0.3 show a positive correlation of Q/I with respect to the continuum intensity. We estimate that the subregion diameter responsible for the excess Q/I signal is on the order of 0.5″-1″. Conclusions. The presented observations and the statistical analysis of Q/I signals at μ = 0.6 complement reported scattering polarization observations as well as simulations. The FSP has proven to be a suitable instrument to measure spatially resolved scattering polarization signals. In the future, a systematic center-to-limb series of observations with subgranular spatial resolution and increased polarimetric sensitivity (<10-3) compared to that in the present study is needed in order to investigate the change in trend with μ that the comparison of our results with the literature suggests.