Fine structure in sunspots. III. Penumbral grains

The properties of penumbral grains (PGs) in a medium-size sunspot are studied from a 4.5 hour observation series acquired on 5 June 1993 at the Swedish Vacuum Solar Telescope, La Palma. The application of an image segmentation procedure and a feature tracking algorithm on a movie of 360 frames yields proper motions, intensities, and lifetimes for a set of 469 PGs. Almost 3/4 of the PGs move toward the umbra and more than 1/4 toward the photosphere. There appears to be a dividing line (DL) in the penumbra, approximately 0.7 of the distance from the umbra to the photosphere, such that most PGs outside this line move toward the photosphere, and those inside move toward the umbra. For inward moving PGs we find a typical proper motion speed of 0.4 km s(-1) and a median lifetime of 29 minutes, for outward moving ones 0.5 km s(-1) and 22 minutes. The average speed of inward moving PGs increases with distance from the umbra with a maximum near the DL. Outward moving PGs have maximum speed near the outer penumbral boundary. The measured instantaneous velocities of individual PGs show only partial agreement with theoretical model predictions. We find much shorter lifetimes than earlier authors, and no pronounced dependence of lifetime on position in the penumbra. We discuss possible reasons for the disagreement with previous results.