Scattering polarization in Sr I 460.7 nm with FSP

Spatial sub-granular fluctuations in the quiet Sun linear Q/I scattering polarization observed

November 21, 2018

by Franziska Zeuner

In the past, observations of the scattering polarization in the photospheric Sr I 460.7 nm line were limited to spatially unresolved spectra close to the limb. With the Fast Solar Polarimeter we observed this line for the first time with a Fabry-Pérot filtergraph at the German Vacuum Tower Telescope on Tenerife, Spain. The goal was to observe spatial fluctuations in the linear Q/I polarization on sub-granular scales in the quiet Sun, which were predicted by 3D radiative transfer simulations. With a statistical approach we found an anti-correlation of the Q/I line core polarization signals with the continuum intensity. This result hints towards higher linear polarization signals in intergranular regions (see Figure 1), which is explained by symmetry-breaking processes at the borders between granules and intergranules. For the future, spatially resolved observations in the Sr I line are expected to increase the sensitivity of magnetic field measurements in the photosphere via Hanle diagnostics. These results are accepted for publication in A&A.

Scatter plots of the polarization signals in Stokes Q/I, U/I and V/I in the Sr I line versus normalized Stokes I continuum intensity. The continuum polarization signals in the left panels are references, which are not correlating with the continuum intensity. In the right panels the polarization signals in the Sr I line core are shown. Only Q/I (linear polarization direction parallel to the solar limb) is showing a correlation as the observed quiet Sun region was located at μ=0.6 towards the north solar limb. Inserts display the function describing a linear fit.

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