Faculae at the poles of the Sun revisited: infrared observations

¹ CICGE, Observatório Astronómico Prof. Manuel de Barros, Fac. Ciências da Universidade do Porto, Alameda Monte da Virgem, 4430-146 Vila Nova de Gaia, Portugal e-mail: jblanco@fc.up.pt
² Institut für Astrophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany e-mail: kneer@astro.physik.uni-goettingen.de

Received: 8 October 2008
Accepted: 17 September 2009

Abstract

Aims. This study extends earlier investigations on faculae and their small-scale magnetic fields near the solar poles (polar faculae – PFe) to measurements of the magnetically sensitive infrared (IR) lines at 1.5 μm, which provide more accurate information about the magnetic field than lines in the visible spectral range.

Methods. PFe were observed with the Tenerife Infrared Polarimeter (TIP II) mounted at the Vacuum Tower Telescope/Observatorio del Teide/Tenerife. Several areas at various heliocentric angles were scanned. Faculae near the solar equator (equatorial faculae – EFe) were also observed for comparison with PFe. The full Stokes vector of the line pair at 1.5 μm was measured. The magnetic field properties were determined (1) from the centre of gravity (COG); (2) with the weak field approximation (WFA); (3) assuming the strong field regime (SFR); and (4) with inversions under the hypothesis of Milne-Eddington (ME) atmospheres. Line-of-sight (LOS) velocities were determined from the COG of I profiles and from the zero-crossing of the V profiles.

Results. The main findings of this work can be divided in five parts: (1) the detected PFe do not harbour sufficient magnetic flux to account for the global flux observed with other methods. (2) Near the solar limb, the apparent, measured transversal field components are most times stronger than the longitudinal components by factors of up to 10 for both PFe and EFe, as found from observations with HINODE SOT. (3) Many PFe indeed harbour kilo-G magnetic fields. Of those, more than 85% possess the same magnetic polarity as the global field. The inclinations γ of the strong fields,  G in the SFR, are compatible with their vertical emergence from the solar surface. (4) The results for weaker fields,  G from ME inversions, indicate a random magnetic field orientation. (5) The velocities from I profiles and V zero-crossings are in average ~0.3 km s^-1 towards observer, for both PFe and EFe. The zero-crossings of V exhibit a large velocity dispersion, of up to 3 km s^-1.