Dynamics of the solar active region finestructure

Abstract

We investigate the dynamical behavior of the finestructure in a sunspot's surroundings and its penumbra from a speckle-reconstructed 60 min time series taken at the 45 cm Dutch Open Telescope (DOT) on La Palma. In the 1 nm spectral window containing the G-band, we determine the area of each feature and its time evolution by means of pattern recognition, particularly adapted to separate bright granular edges from inter-granular G-band bright points (BP). The evolution of each individual BP shows a stronger variation of the area than of the intensity. We analyze the horizontal motions of BP as a function of their distance from the sunspot center. Within a 6 Mm ring around the outer sunspot border, most BP (4/5) move radially outwards; they are faster than the minority (1/5) of inward moving BP. The difference of both velocities indicates a radial outward drift which decreases from about 0.3 km s-1 at the outer penumbral border to zero at about 20 Mm distance (28″) from the sunspot center; a spatial range that we interpret as the extension of the sunpot "moat". This finding supports the idea of giant rolls in deep layers measured by helio-seismic tomography and predicted by theory. Inside the penumbra, we find a 4/5 majority of penumbral bright structures (PBS) to move inwards with a mean velocity of 0.8 km s-1. The 1/5 minority of outward moving PBS is almost entirely located in the outer penumbra; their mean velocity of 0.8 km s-1 is equally found for penumbral dark structures (PDS) in the outer penumbra, in agreement with penumbral MHD models.