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Astron. Astrophys. 328, 689-694

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Fine structure in sunspots

II. Intensity variations and proper motions 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 25 March 1997 / Accepted 19 June 1997


Temporal intensity variations of umbral dots (UDs) and dark nuclei (DNs), and proper motions of UDs, were analyzed in a 4 1/2 hour time series of high resolution white light images of the umbra in a medium-size sunspot (NOAA 7519). The observations were made on 5 June 1993 at the Swedish Vacuum Solar Telescope, La Palma. An identification and tracking algorithm was applied to UDs observed in a destretched movie of 360 frames. In total, 662 UDs were tracked, and their intensities, positions, and proper motions were measured. Power spectra of temporal intensity variations of UDs and DNs were computed, and several typical periods were found. The histogram of time-averaged intensities of UDs has two maxima; the UDs belonging to the brighter part of the population are located mostly at or near the umbral-penumbral boundary. The number of UDs decreases with increasing magnitude of the proper motion velocity. Speeds of UDs are grouped at 100 and 400 m/s. The observed spatial distribution of UDs with different proper motion velocities is found to be in contradiction to the generally accepted idea of moving ``peripheral'' and stationary ``central'' UDs. Both ``fast'' and ``slow'' UDs are present in all parts of the umbra. Thus velocity does not appear to be a good criterion for separating UDs into ``peripheral'' and ``central'' ones.

Key words: Sun: sunspots - physical data and processes: convection

Send offprint requests to: M. Sobotka

© European Southern Observatory (ESO) 1997

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