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Astron. Astrophys. 328, 682-688 (1997)

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Fine structure in sunspots

I. Sizes and lifetimes of umbral dots

Michal Sobotka 1, Peter N. , Brandt 2, George W. and Simon 3

1 Astronomical Institute, Academy of Sciences of the Czech Republic, CZ-25165 Ondejov, Czech Republic
2 Kiepenheuer-Institut, D-79104 Freiburg, Germany
3 PL/GPSS, Sacramento Peak, Sunspot, NM 88349, USA

Received 17 March 1997 / Accepted 19 June 1997


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 [FORMULA]) and long-lived (lifetime [FORMULA] 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.

Key words: Sun: sunspots – physical data and processes: convection – techniques: image processing

Send offprint requests to: M. Sobotka


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© European Southern Observatory (ESO) 1997

Online publication: March 26, 1998