Fine structure in sunspots - IV. Penumbral grains in speckle reconstructedimages

The properties of penumbral grains (PGs) in a large regular sunspot are stu died 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, intensitie s, and lifetimes for a set of 1058 PGs with lifetimes longer than 10 min. A bout 54% of the PGs move toward the umbra and 46% toward the photosphere. T he 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 an d 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 s peed are 31 min and 0.75 km s(-1). These measurements confirm the previous results published by Sobotka et al. (1999a).