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Some properties of sunspot umbral dots
A. Tritschler and
Received 1 April 1996 / Accepted 16 October 1996
High resolution Stokes-I spectra of the Zeeman-sensitive lines at 846.85 nm and 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 ( 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 846.85 nm spectra indicates that the magnetic field strength within the umbral features is not significantly reduced neither for centrally ( 3 %) nor for peripherically located UDs (1-2 %). The 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
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998