On the structure of polar faculae on the Sun

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 Fe I 6301.5 and 6302.5 Å and Fe 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 0.25 arcsec for broadband images and 0.5 arcsec for magnetograms. The application of singular value decomposition yielded a polarimetric detection limit of | V|≈2 × 10^-3I_c. We find that PFe, of size of 1 arcsec 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_FT≈-0.4 km s^-1 (towards the observer).