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G.W. Simon (Air Force Research Lab and National Solar Observatory), M. Sobotka (Astronomical Institute, Academy of Sciences, Czech Republic), P.B. Brandt (Kiepenheuer-Institut)
The properties of penumbral grains (PGs) in a medium-size sunspot are studied from a 4.5 h observation series acquired on 1993 June 5 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 the inward moving PGs we find a typical proper motion speed of 0.4 km/s and a median lifetime of 29 minutes, for the outward moving ones 0.5 km/s and 22 minutes. The average speed of inward moving PGs increases with distance from the umbra and has a maximum near the DL. Outward moving PGs have maximum speed near the outer penumbral boundary. Instantaneous velocities of individual PGs were measured to compare them 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 in results, and make some comments on the differences between human and computer selection and tracking of features.
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