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Astron. Astrophys. 321, 643-651 (1997)

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Some properties of sunspot umbral dots

A. Tritschler and W. Schmidt

Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, D-79104 Freiburg, Germany

Received 1 April 1996 / Accepted 16 October 1996


High resolution Stokes-I spectra of the Zeeman-sensitive lines at [FORMULA]  846.85 nm and [FORMULA]  630.25 nm were used to determine the magnetic field strength and the brightness temperature of a simple, relatively symmetric sunspot umbra and its umbral dots (UDs).

We find a decrease in brightness temperature for central ([FORMULA] 30 %) and peripheral UDs (20-25 %) compared to the surrounding quiet sun. This corresponds to a difference in temperature of about 1600 K and 1200-1400 K respectively.

The magnetic field strength within the umbra is derived from the Zeeman splitting of the Stokes-I profile. Field strengths within the umbra vary between 1500 and 3000 G. Further analysis of the [FORMULA]  846.85 nm spectra indicates that the magnetic field strength within the umbral features is not significantly reduced neither for centrally ([FORMULA]  3 %) nor for peripherically located UDs (1-2 %). The [FORMULA]  630.25 nm line yields an enhanced weakening of field strength of 7 % for an isolated located UD.

The measured properties of UDs do not vary significantly during a 40 min observation sequence.

Key words: solar activity – sunspots – umbral dots


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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998