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

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6. Conclusions

We have empirically determined brightness temperatures and magnetic field strengths of small-scale umbral features. The results of our observations can be summarized as follows:

  1. From the absence of a significant reduction of field strength in the UDs ([FORMULA] 3 %) we concluded that UDs are "magnetically invisible" in the regions where the Fe I  [FORMULA]  864.8 nm line is formed.
  2. We find a decrease of brightness temperature in UDs by [FORMULA] 30 % for CUDs and by 20-25 % for PUDs compared to the surrounding quiet photosphere
  3. Umbral dots show an excess of brightness temperature by [FORMULA] 300 K relative to the umbral background.
  4. The UDs are still visible in the height of the line core, but with reduced intensity contrast.
  5. The observed UDs gradually fade away during the 40 min time interval of our measurements.

Our derived continuum contrasts are in agreement with other observations (Koutchmy & Adjabshirzadeh 1981; Lites et al. 1991; Sobotka et al. 1992a, 1992b; Sobotka et al. 1993). The investigated UDs show brightness temperatures of more than 1000 K below those of the photosphere. Grossmann-Doerth et al. (1986) find a temperature decrease of up to 1000 K in UDs. In contrast, results from two-colour analysis (Beckers & Schröter 1969, Koutchmy & Adjabshirzadeh 1981) or inhomogeneous modelling of sunspot umbrae (Koutchmy & Adjabshirzadeh 1981; Pahlke & Wiehr 1990) indicate photospheric or nearly photospheric temperatures of UDs.

The Fe I  [FORMULA]  864.8 nm line used in this work forms somewhat higher in the solar atmosphere compared to the Fe I  [FORMULA]  630.25 nm line and other lines used for spectroscopic investigations of UDs. Although other recent investigations of UDs yield higher magnetic field reductions (Balthasar & Schmidt 1993; Wiehr & Degenhardt 1993; Balthasar & Schmidt 1994), our results are in qualitative agreement with these observations, if they are seen as a consequence of different formation heights of the spectral lines inside and outside the umbral features and the height dependence of the magnetic field.

Our findings support the idea that UDs are phenomena of the deep umbra (Degenhardt & Lites 1993). They are best visible only at the continuum level of the umbral photosphere and are rather inconspicuous in the regions of line formation, where all measurements of magnetic field strength and material motions are made.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998