Inclination of magnetic fields and flows in sunspot penumbrae

¹ The Institute for Solar Physics of the Royal Swedish Academy of Sciences, AlbaNova University Center, 10691 Stockholm, Sweden e-mail: [kai;scharmer;dan;mats]@astro.su.se
² Lockheed Martin Solar and Astrophysics Lab, 3251 Hanover St., Palo Alto, CA 94304, USA e-mail: berger@lmsal.com

Received: 11 January 2005
Accepted: 22 February 2005

Abstract

An observational study of the inclination of magnetic fields and flows in sunspot penumbrae at a spatial resolution of  is presented. The analysis is based on longitudinal magnetograms and Dopplergrams obtained with the Swedish 1-m Solar Telescope on La Palma using the Lockheed Solar Optical Universal Polarimeter birefringent filter. Data from two sunspots observed at several heliocentric angles between 12° and 39° were analyzed. We find that the magnetic field at the level of the formation of the Fe i-line wing (630.25 nm) is in the form of coherent structures that extend radially over nearly the entire penumbra giving the impression of vertical sheet-like structures. The inclination of the field varies up to 45° over azimuthal distances close to the resolution limit of the magnetograms. Dark penumbral cores, and their extensions into the outer penumbra, are prominent features associated with the more horizontal component of the magnetic field. The inclination of this dark penumbral component – designated B – increases outwards from approximately 40° in the inner penumbra such that the field lines are nearly horizontal or even return to the solar surface already in the middle penumbra. The bright component of filaments – designated A – is associated with the more vertical component of the magnetic field and has an inclination with respect to the normal of about 35° in the inner penumbra, increasing to about 60° towards the outer boundary. The magnetogram signal is lower in the dark component B regions than in the bright component A regions of the penumbral filaments. The measured rapid azimuthal variation of the magnetogram signal is interpreted as being caused by combined fluctuations of inclination and magnetic field strength. The Dopplergrams show that the velocity field associated with penumbral component B is roughly aligned with the magnetic field while component A flows are more horizontal than the magnetic field. The observations give general support to fluted and uncombed models of the penumbra. The long-lived nature of the dark-cored filaments makes it difficult to interpret these as evidence for convective exchange of flux tubes. Our observations are in broad agreement with the two component model of Bellot Rubi et al. (2003), but do not rule out the embedded flux tube model of Solanki & Montavon (1993).