EDP Sciences
Free access
Volume 440, Number 1, September II 2005
Page(s) 345 - 356
Section The Sun
DOI http://dx.doi.org/10.1051/0004-6361:20042042

A&A 440, 345-356 (2005)
DOI: 10.1051/0004-6361:20042042

Spectropolarimetry of a sunspot at disk centre

M. Sánchez Cuberes1, K. G. Puschmann2 and E. Wiehr2

1  Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
    e-mail: msanchez@aip.de
2  Universitäts-Sternwarte, Geismarlandstr. 11, 37083 Göttingen, Germany
    e-mail: [kgp;ewiehr]@uni-sw.gwdg.de

(Received 21 September 2004 / Accepted 3 February 2005 )

The magnetic, thermal and velocity structure of a sunspot at the solar disk centre ( $\theta=2^{\circ}$) is investigated by inverting the full Stokes profiles of three infrared lines. A single magnetic component atmosphere is assumed with height gradients of the physical quantities. Since the line-of-sight (LOS) is perpendicular to the solar surface, differential optical or projection effects do not interfere, as often is the case for the usual observations at oblique LOS. We find a symmetric configuration of the field and flow and the downward motion that increases with radial distance by up to 3 km s-1 near the outer penumbral border. The magnetic field is found to be highly axially symmetric without any indication of azimuthal vortices. A tight relation between field strength and inclination is obtained with a gradient of $35^\circ$/1000 G independent of height. The penumbra shows "spines" hosting a pronounced negative correlation between field strength and inclination in the sense that steeper and stronger magnetic fields are related to brightenings in the line cores but not in the continuum. We discuss the dependence of the obtained results on different assumptions of parasitic light, and present indications of its overestimation by the inversion code.

Key words: Sun: photosphere -- Sun: sunspots -- techniques: polarimetric -- Sun: magnetic fields -- line: profiles

© ESO 2005