Inclination of magnetic fields and flows in sunspot penumbrae
Abstract.
An empirical determination of the inclination of magnetic fields and flows in sunspot penumbrae, at a spatial resolution of 0."2, 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 12degr and 39degr were analyzed. We find that the magnetic field at the level of the formation of the FeI-line wing (630.25nm) 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 can change with up to 45degr over azimuthal distances close to the resolution limit of the magnetograms. In these magnetograms, dark penumbral cores and their extensions into the outer penumbra, are prominent features which are associated with the more horizontal component of the magnetic field. The inclination of this penumbral component -- designated B -- increases outwards from approximately 40degr 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 is associated with the more vertical component of the magnetic field and has an inclination with respect to the normal of about 35$\degr$ in the inner penumbra, increasing to about 60degr towards the outer boundary. We designate this penumbral structure as component A. The analysis also shows that the magnetogram signal is lower in the dark parts than in the bright parts 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. In the Dopplergrams, we find that the velocity field associated with the penumbral component B (thus locally dark filaments) is roughly aligned with the magnetic field while component A is associated with flows that 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 Rubio et al. (2003), but do not rule out the embedded flux tube model of Solanki & Montavon (1993).