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Astron. Astrophys. 328, 682-688

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Fine structure in sunspots

I. Sizes and lifetimes of umbral dots

Michal Sobotka1, Peter N. Brandt2, and George W. Simon3

1Astronomical Institute, Academy of Sciences of the Czech Republic, CZ-25165 Ondrejov, Czech Republic
2Kiepenheuer-Institut, D-79104 Freiburg, Germany
3PL/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 $\gt
0\hbox{$.\!\!^{\prime\prime}$}4$) 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.

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

Send offprint requests to: M. Sobotka

© European Southern Observatory (ESO) 1997

Online publication: November 24, 1997
Last change: March 26, 1998