On the properties of faculae at the poles of the Sun

¹ Institut für Astrophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany e-mail: blanco@astro.physik.uni-goettingen.de
² Central Astronomical Observatory at Pulkovo, 196140 St. Petersburg, Russia
³ Max-Plank-Institut für Sonnensystemforschung, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany

Received: 27 April 2007
Accepted: 23 July 2007

Abstract

Context.Faculae at the poles of the Sun, or polar faculae (PFe), take part in the solar magnetic cycle. Their occurrence maximum is shifted by 5–6 years with respect to the sunspot cycle. PFe are stable phenomena, with lifetimes of several hours to days, and harbour magnetic fields of kilo-Gauss strength. Yet their role for the global magnetic field at the solar poles is unknown.

Aims.To contribute to the knowledge of the physical properties of PFe and to the understanding of their role in the global magnetism of the Sun.

Methods.PFe were observed on 21–24 August 2005 with the Vacuum Tower Telescope at the Observatorio del Teide. The “Göttingen” Fabry-Perot spectrometer was used with a Stokes V polarimeter to scan the Fe i 6173 Å line (Landé factor g = 2.5) and the Hα line in two-dimensional fields of view (FOVs). A large observational coverage of the polar caps was obtained. The data were analysed with speckle methods. Magnetic field strengths were determined with the weak field approximation, with the approximation of the strong field regime, and with the centre of gravity (COG) method. Velocities were measured with the COG method and from the zero-crossing of the Stokes V profiles.

Results.PFe show a decrease of the continuum and broadband intensity contrast towards the disc centre and no decrease of contrast towards the limb, similar to as faculae in active regions near the equator. Extrapolating from the observed FOVs to the total areas of PF occurrence around the solar poles, we find 4 120 PFe in the northern polar cap and, asymmetrically to this number, 1 250 PFe near the south pole. The total area coverages by PFe are ~7.610⁸ km² and ~3.410⁸ km² near the solar north and south poles, respectively. Some of the PFe exhibit magnetic polarities opposite to the global polarity at the time of observation. The resulting total magnetic fluxes in PFe fall short by an order of magnitude from those found in the literature for the fluxes at the polar caps. This also holds if we include magnetic structures which are not related to brightenings. We conclude that with the present spatial resolution of (FWHM), PFe represent the “large-scale” end of a distribution of unipolar strands near the solar poles. The velocities in PFe show amplitudes of 2 km s^-1, with systematic up-flows in the Stokes I profile, but no average velocity measured in the V zero-crossings.