Spatial variations of the Sr I 4607 Å scattering polarization peak

¹ Istituto Ricerche Solari Locarno, IRSOL, associated to Università della Svizzera italiana, Locarno, Switzerland
e-mail: mbianda@irsol.ch
² Kiepenheuer Institut für Sonnenforschung, Freiburg, Germany
³ ETHZ, Zurich, Switzerland

Received: 29 December 2017
Accepted: 13 February 2018

Abstract

Context. The scattering polarization signal observed in the photospheric Sr I 4607 Å line is expected to vary at granular spatial scales. This variation can be due to changes in the magnetic field intensity and orientation (Hanle effect), but also to spatial and temporal variations in the plasma properties. Measuring the spatial variation of such polarization signal would allow us to study the properties of the magnetic fields at subgranular scales, but observations are challenging since both high spatial resolution and high spectropolarimetric sensitivity are required.

Aims. We aim to provide observational evidence of the polarization peak spatial variations, and to analyze the correlation they might have with granulation.

Methods. Observations conjugating high spatial resolution and high spectropolarimetric precision were performed with the Zurich IMaging POLarimeter, ZIMPOL, at the GREGOR solar telescope, taking advantage of the adaptive optics system and the newly installed image derotator.

Results. Spatial variations of the scattering polarization in the Sr I 4607 Å line are clearly observed. The spatial scale of these variations is comparable with the granular size. Small correlations between the polarization signal amplitude and the continuum intensity indicate that the polarization is higher at the center of granules than in the intergranular lanes.