Fine structure in sunspots I. Sizes and lifetimes of umbral dots

The analysis of a 4 1/2 hour series of high resolution white light observations of the umbra in a medium-size sunspot (NOAA 7519, observed on 5 June 1993 at the Swedish Vacuum Solar Telescope, La Palma) is described. An automatic identification and tracking algorithmwas applied to umbral dots (UDs) observed in a destretched movie of 360 frames. In total, 662 UDs were tracked and their filling factor, sizes, and lifetimes were measured. It was found that large (diameter > 0. ′′4) and long-lived (lifetime > 10 minutes) UDs appear mostly in regions with enhanced umbral diffuse background intensity. UDs do not have a “typical” size. Their number rapidly increases with decreasing diameter down to the resolution limit. Similarly, UDs do not have a “typical” lifetime, and their number rapidly increases with decreasing lifetime. UDs with lifetimes below 10 minutes represent about 2/3 of the population; the median lifetime is 5.9 minutes.