On the Fine Structure and Formation of Sunspot Penumbrae

Abstract

Recent high-resolution observations with the 1mSwedish Solar Telescope (SST) on La Palma reveal the fine sub- structure of penumbral filaments and newproperties of their dynamics.These findings provide both the basis and con- straints for novel models of the penumbra.We present new observations of a large isolated sunspot near Sun center obtainedwith the SSTin 2006.Our data, taken simultaneously in the 43058G-band and 43648continuumbandpasses and compiled in high-cadence movies, confirm the previous results and reveal new features of penumbral filament dynamics.We find that individual filaments are cylindrical helices with the apparent properties of vortex tubes exhib- iting flow patterns similar to kinked flux tubes. Measured pitch/radius ratios of helical filaments indicate their dy- namic stability. We propose a mechanism that explains the fine structure of penumbral filaments, their observed dynamics, and their formation process in association with sunspot properties. The mechanism assumes that the umbra itself is a dense conglomerate of twisted, interlaced flux tubeswith peripheral filaments branching out fromthe ‘‘trunk’’ at different heights due to ongoing reconnection processes and arcing downward to the photosphere. The twist of individual filaments, and the resulting distribution of magnetic fields and temperature, is due to the onset of the well- known screw pinch instability, the parameters of which can be measured from our data.