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 algorithm was 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 > 0farcs 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.
Astronomy and Astrophysics
- Pub Date:
- December 1997
- SUN: SUNSPOTS;
- PHYSICAL DATA AND PROCESSES: CONVECTION;
- TECHNIQUES: IMAGE PROCESSING