Fine structure in sunspots

The properties of penumbral grains (PGs) in a large regular sunspot are studied from a 70 min sequence of G-band images acquired on 20 September 1999 at the Dutch Open Telescope, La Palma. The frames were processed using the speckle masking algorithm, resulting in an almost diffraction-limited time series (30 s cadence), basically free of atmospheric distortions. Applying feature tracking to a movie of 140 frames yields proper motions, intensities, and lifetimes for a set of 1058 PGs with lifetimes longer than 10 min. About 54% of the PGs move toward the umbra and 46% toward the photosphere. The inward-moving PGs are located mostly in the inner penumbra (up to 0.6 of the distance from the umbra to the photosphere). Their average lifetime and median speed are 50 min and 0.52 km s-1. Most of the outward-moving PGs are observed in the outer penumbra and their average lifetime and median speed are 31 min and 0.75 km s-1. These measurements confirm the previous results published by Sobotka et al. ([CITE])