Volume 596, December 2016
GREGOR first results
|Number of page(s)||13|
|Published online||30 November 2016|
Magnetic fields of opposite polarity in sunspot penumbrae
1 Kiepenheuer Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany
2 Instituto de Astrofísica de Canarias, c/Vía Láctea s/n, 38205 La Laguna, Spain
3 Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna (Tenerife), Spain
4 Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
5 Max-Planck-Institut für Sonnensystemforschung, Justus von Liebig Weg 3, 37077 Göttingen, Germany
6 Institut für Astrophysik, Friedrich Hund Platz 1, 37077 Göttingen, Germany
7 Astronomical Institute, Academy of Sciences of the Czech Republic, Fričova 298, 25165 Ondřejov, Czech Republic
8 Kyung Hee University, Yongin, Gyeonggi-Do, 446 701 Republic of Korea
9 USRA Huntsville, 6767 Madison Pike # 450, Huntsville, AL 35806, USA
Received: 29 February 2016
Accepted: 1 August 2016
Context. A significant part of the penumbral magnetic field returns below the surface in the very deep photosphere. For lines in the visible, a large portion of this return field can only be detected indirectly by studying its imprints on strongly asymmetric and three-lobed Stokes V profiles. Infrared lines probe a narrow layer in the very deep photosphere, providing the possibility of directly measuring the orientation of magnetic fields close to the solar surface.
Aims. We study the topology of the penumbral magnetic field in the lower photosphere, focusing on regions where it returns below the surface.
Methods. We analyzed 71 spectropolarimetric datasets from Hinode and from the GREGOR infrared spectrograph. We inferred the quality and polarimetric accuracy of the infrared data after applying several reduction steps. Techniques of spectral inversion and forward synthesis were used to test the detection algorithm. We compared the morphology and the fractional penumbral area covered by reversed-polarity and three-lobed Stokes V profiles for sunspots at disk center. We determined the amount of reversed-polarity and three-lobed Stokes V profiles in visible and infrared data of sunspots at various heliocentric angles. From the results, we computed center-to-limb variation curves, which were interpreted in the context of existing penumbral models.
Results. Observations in visible and near-infrared spectral lines yield a significant difference in the penumbral area covered by magnetic fields of opposite polarity. In the infrared, the number of reversed-polarity Stokes V profiles is smaller by a factor of two than in the visible. For three-lobed Stokes V profiles the numbers differ by up to an order of magnitude.
Key words: sunspots / Sun: infrared / Sun: photosphere / Sun: magnetic fields
© ESO, 2016
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