Fast Solar Polarimeter (Prototype): Preliminary Results of Stokes Measurements in the Sr I 4607 Å Line at VTT/TESOS

ZEUNER, F.; FELLER, A.; IGLESIAS, F. A; SOLANKI, S.K.

Florencia

Workshop; Solar Polarization Workshop 8; 2016

Scattering polarization measurements at spatial scales in the 0.1− 1"regime are a very promising complementary diagnostic for the Sun?s atmosphere andmagnetism at small spatial scales (e.g., Trujillo Bueno et al. 2004). So far, for the Sr iline at 4607.3 Å, which exhibits a large scattering polarization signal (Stenflo et al.1997; Gandorfer 2002), only theoretical predictions for the strength and spatial distribution of linear polarization signals at sub-arcsecond scales close to disk center aremade available by Trujillo Bueno & Shchukina (2007), whereas observational feedbackis still very rare.Here, we present preliminary results of two observational campaigns with theprototype of the Fast Solar Polarimeter (FSP, Iglesias et al. 2016) attached to the TESOSfiltergraph, which is located at the German Vacuum Tower Telescope (VTT) in Tenerife,Spain.We measured the center-to-limb variation of spatially averaged Stokes Q/I profiles in the Sr i line at 11 wavelength positions to check for systematic errors and tocompare with previous results by other instruments. We find that our results are inagreement with previous findings reported in the literature (i.e., Stenflo et al. 1997).In May 2015, we observed the Sr i line in the quiet Sun at µ = 0.6 from the northsolar limb with a spatial sampling of 0.08′′pixel−1and noise levels significantly below1% per pixel for linear polarized signals with 1.25 s integration time. To obtain lowernoise levels (< 0.1%), while conserving sufficient spatial resolution to resolve solargranulation, we carefully used spatial, spectral, and temporal averaging, as well as aMOMFBD restoration (see van Noort et al. 2005). We compare our findings in the Sr iline with the neighboring, but not scattering sensitive Fe i line at 4607.6 Å.Our statistical analysis of Stokes Q/I signals in the line core of Sr i reveals that thepolarization amplitude is correlating with the intensity of the continuum image. We findstronger linear polarimetric signals corresponding to intergranular lanes of the Stokes Iimage, which is in contrast to the findings of Malherbe et al. (2007) at µ = 0.3.The results have been published as an article entitled ?Detection of spatially structured scattering polarization of Sr i 4607.3 Å with the Fast Solar Polarimeter? (Zeuneret al. 2018).