Astronomy & Astrophysics

Free access article

A&A 434, 317-327 (2005)
DOI: 10.1051/0004-6361:20042001

Polarimetry of sunspot penumbrae with high spatial resolution

N. Bello González¹, O. V. Okunev^{1, 2}, I. Domínguez Cerdeña¹, F. Kneer¹ and K. G. Puschmann¹

¹ Universitäts-Sternwarte, Geismarlandstr. 11, 37083 Göttingen, Germany
e-mail: nazaret@uni-sw.gwdg.de
² Central Astronomical Observatory at Pulkovo, 196140 St. Petersburg, Russia

(Received 13 September 2004 / Accepted 10 December 2004 )

Abstract
We present two-dimensional high-spatial-resolution spectropolarimetric observations of sunspot penumbrae. They were obtained in April 2002 and May 2003 with the "Göttingen" Fabry-Pérot spectrometer at the Vacuum Tower Telescope at the Observatorio del Teide (Tenerife). Speckle methods were applied for image reconstruction which resulted in a spatial resolution of 0 $\farcs$ 5 in the magnetograms of the penumbrae. We analysed Stokes I and V profiles of the $\ion{ii}$ 6149 Å line, which exhibits no instrumental Stokes $Q/U\rightarrow V$ crosstalk, and of the $\ion{i}$ line pair at 6302 Å. The main results are the following: 1) on scales larger than 0 $\farcs$ 5, the intensity pattern of penumbrae stays the same in the continuum and core images of the 6301.5 Å line, which stem from 0 km and 300 km (above $\tau_5=1$ ), respectively. Yet at scales of 0 $\farcs$ 5 and smaller the pattern in the two spectral features is clearly different. 2) On the limb side of sunspots the Evershed flow is carried by dark filaments and on the centre side by bright features and their somewhat weakened tails. We explain this with a picture in which the velocity of hot rising gas is best seen on the centre side, while on the limb side the horizontal outward and possibly downward flows are seen when the gas has cooled down. 3) The un-combed structure of the magnetic field is confirmed. On the limb side, the more horizontal fields coincide with dark fibrils or with diffuse intensity structures. Generally, the more horizontal fields are located at the positions of strong outflows. 4) Strong line-of-sight components of the magnetic fields are not found in bright filaments but in dark structures, somewhat displaced from the darkest parts. Their positions do not coincide with those of the strongest velocity fields. In general, our results are compatible with the picture of low lying flow channels coincident with the horizontal magnetic field, or possibly emerging and diving down into sub-photospheric layers, like a "sea serpent". Some further dynamic phenomena are discussed, which demonstrate the richness of processes in penumbrae, and reveal unexpected properties.

Key words: Sun: sunspots -- Sun: magnetic fields -- techniques: high angular resolution