Astronomy & Astrophysics

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Issue		A&A Volume 425, Number 1, October I 2004
Page(s)		321 - 331
Section		The Sun
DOI		10.1051/0004-6361:20041120

A&A 425, 321-331 (2004)
DOI: 10.1051/0004-6361:20041120

On the structure of polar faculae on the Sun

O. V. Okunev^{1, 2} and F. Kneer¹

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

(Received 20 April 2004 / Accepted 25 May 2004 )

Abstract
Faculae on the polar caps of the Sun, in short polar faculae (PFe), are investigated. They take part in the magnetic solar cycle. Here, we study the fine structures of PFe, their magnetic fields and their dynamics on short time scales. The observations stem from several periods in 2001 and 2002. They consist of spectropolarimetric data (Stokes I and V) taken in the $\ion{i}$ 6301.5 and 6302.5 Å and $\ion{ii}$ 6149.3 Å lines with the Gregory-Coudé Telescope (GCT) and the Vacuum Tower Telescope (VTT) at the Observatorio del Teide on Tenerife. At the VTT, the "Göttingen" two-dimensional Fabry-Perot spectrometer was used. It allows image reconstruction with speckle methods resulting in spatial resolution of approximately 0 $\farcs$ 25 for broadband images and 0 $\farcs$ 5 for magnetograms. The application of singular value decomposition yielded a polarimetric detection limit of $\vert V\vert\approx2$ $\times$ $10^I_{\rm c}$ . We find that PFe, of size of 1´´ or larger, possess substantial fine structure of both brightness and magnetic fields. The brightness and the location of polar facular points change noticeably within 50 s. The facular points have strong, kilo-Gauss magnetic fields, they are unipolar with the same polarity as the global, poloidal magnetic field. The ambient areas, however, exhibit weak flux features of both polarities, as in the quiet Sun near disk center. Strong upflows of 1 km s ^-1 are detected in the intensity profiles of PFe. The zero-crossings of the Stokes V profiles yield an average velocity of the magnetized plasma of $v_{{\rm FT}}\approx-0.4$ km s ^-1 (towards the observer).

Key words: Sun: faculae, plages -- Sun: magnetic fields -- techniques: high angular resolution

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