Abstract
Photographic spectra of the umbra of a sunspot (1971, August 24, Rome No. 6205) around 6150 Å show fine bright threads which were identified as the spectra of a lightbridge, of the bright end of a penumbral filament and of umbral dots, respectively. It was found, in agreement with the results of other authors, that the magnetic field in bright structures is considerably weaker than in dark umbral material. Analysis of line profiles of Fe ii 6149.2 Å in umbral dots indicates (a) a fieldstrength reduced by a factor ≈2 compared to the surroundings, (b) an outflow with v≈3.0 km s−1 relative to the penumbra and (c) possibly photospheric temperatures in umbral dots.
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Mitteilungen aus dem Fraunhofer Institut Nr. 115.
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Kneer, F. On some characteristics of umbral fine structure. Sol Phys 28, 361–367 (1973). https://doi.org/10.1007/BF00152306
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DOI: https://doi.org/10.1007/BF00152306