Observations on spatial variations of the Sr~{sc i} 4607~AA~scattering polarization signals at different limb distances with ZIMPOL. (arXiv:1908.03366v1 [astro-ph.SR])

Observations on spatial variations of the Sr~{sc i} 4607~AA~scattering polarization signals at different limb distances with ZIMPOL. (arXiv:1908.03366v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Dhara_S/0/1/0/all/0/1">Sajal Kumar Dhara</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Capozzi_E/0/1/0/all/0/1">Emilia Capozzi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gisler_D/0/1/0/all/0/1">Daniel Gisler</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bianda_M/0/1/0/all/0/1">Michele Bianda</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ramelli_R/0/1/0/all/0/1">Renzo Ramelli</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Berdyugina_S/0/1/0/all/0/1">Svetlana Berdyugina</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Alsina_E/0/1/0/all/0/1">Ernest Alsina</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Belluzzi_L/0/1/0/all/0/1">Luca Belluzzi</a>

The Sr~{sc i} 4607~AA spectral line shows one of the strongest scattering
polarization signals in the visible solar spectrum. The amplitude of this
polarization signal is expected to vary at granular spatial scales, due to the
combined action of the Hanle effect and the local anisotropy of the radiation
field. Observing these variations would be of great interest because it would
provide precious information on the small-scale activity of the solar
photosphere. At present, few detections of such spatial variations have been
reported. This is due to the difficulty of these measurements, which require
combining high spatial ($sim$ 0.1″), spectral ($leq$ 20 mAA), and temporal
resolution (< 1 min) with increased polarimetric sensitivity ($sim$ 10$^-$$^4$). Aims. We aim to detect spatial variations at granular scales of the scattering polarization peak of the Sr~{sc i} 4607~AA line at different limb distances, and to study the correlation with the continuum intensity. Methods.Using the Zurich IMaging POLarimeter (ZIMPOL) system mounted at the GREGOR telescope and spectrograph in Tenerife, Spain, we carried out spectro-polarimetric measurements to obtain the four Stokes parameters in the Sr~{sc i} line at different limb distances, from $mu=0.2$ to $mu=0.8$, on the solar disk. Results.Spatial variations of the scattering polarization signal in the Sr~{sc i} 4607~AA line, with a spatial resolution of about 0.66", are clearly observed at every $mu$. The spatial scale of these variations is comparable to the granular size. A statistical analysis reveals that the linear scattering polarization amplitude in this Sr~{sc i} spectral line is positively correlated with the intensity in the continuum, corresponding to the granules, at every $mu$.

The Sr~{sc i} 4607~AA spectral line shows one of the strongest scattering
polarization signals in the visible solar spectrum. The amplitude of this
polarization signal is expected to vary at granular spatial scales, due to the
combined action of the Hanle effect and the local anisotropy of the radiation
field. Observing these variations would be of great interest because it would
provide precious information on the small-scale activity of the solar
photosphere. At present, few detections of such spatial variations have been
reported. This is due to the difficulty of these measurements, which require
combining high spatial ($sim$ 0.1″), spectral ($leq$ 20 mAA), and temporal
resolution (< 1 min) with increased polarimetric sensitivity ($sim$
10$^-$$^4$). Aims. We aim to detect spatial variations at granular scales of
the scattering polarization peak of the Sr~{sc i} 4607~AA line at different
limb distances, and to study the correlation with the continuum intensity.
Methods.Using the Zurich IMaging POLarimeter (ZIMPOL) system mounted at the
GREGOR telescope and spectrograph in Tenerife, Spain, we carried out
spectro-polarimetric measurements to obtain the four Stokes parameters in the
Sr~{sc i} line at different limb distances, from $mu=0.2$ to $mu=0.8$, on
the solar disk. Results.Spatial variations of the scattering polarization
signal in the Sr~{sc i} 4607~AA line, with a spatial resolution of about
0.66″, are clearly observed at every $mu$. The spatial scale of these
variations is comparable to the granular size. A statistical analysis reveals
that the linear scattering polarization amplitude in this Sr~{sc i} spectral
line is positively correlated with the intensity in the continuum,
corresponding to the granules, at every $mu$.

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