Observations of dark-cored filaments in sunspot penumbrae

The Institute for Solar Physics of the Royal Swedish Academy of Sciences, AlbaNova University Center, 106 91 Stockholm, Sweden e-mail: kai@langhanse.at; [scharmer;dan;mats]@astro.su.se  

Received: 15 March 2006
Accepted: 16 December 2006

Abstract

Context. The recent discovery of dark-cored penumbral filaments suggests that we are resolving the building blocks of sunspot penumbrae. Their properties are largely unknown but provide important clues to understanding penumbral fine structure.

Aims.Our observations provide new constraints for the different scenarios put forward to explain the structure of sunspot penumbrae.

Methods.We present an analysis of dark-cored penumbral filaments, based on intensity filtergrams (G-band, continuum and  H line wing), magnetograms and Dopplergrams, obtained at heliocentric distances between and .

Results.In general, the visibility of dark cores degrades with increasing heliocentric distance. Based on  H wing images we conclude that this is due to a geometrical 3D-effect and not due to a simple formation height effect. Only in the center-side penumbra are dark-cored filaments visible at all observed heliocentric distances. We observe that dark-cored filaments frequently split in the umbra, forming a Y-shape that disappears after a few minutes, leaving a shortened filamentary structure and a bright dot in the umbra. The dark-cored filaments have life times ≥ min. The dark cores are related to a much weaker and a more horizontal magnetic field than their lateral brightenings. Where the dark-cored filaments appear in the umbra, the magnetic field is inclined by 40° with respect to the solar surface normal for both the dark core and the bright edges. With increasing distance from the umbra, the magnetic field inclination in the dark cores increases rapidly within a few thousand km. Both the magnetic field strength and inclination in the lateral brightenings show very small variations with spot-center radial distance. The velocity field possesses a strong horizontal component within the dark cores. The absolute line-of-sight (LOS) velocity is larger within the dark cores than in their lateral brightenings. The Evershed flow apparently is present primarily in the dark cores.