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

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1. Introduction

The study of umbrae and the variety of their structural details play a key role in the unsolved issue of energy transport in sunspots. Umbral dots carry information on the magnetic structure and the thermal state of the near-surface layers of the umbra. They contribute up to 40 % to the umbral intensity at visible wavelengths (Adjabshirzadeh & Koutchmy 1983) and therefore represent a source of the observed umbral brightness. Reviews of the relevant observable parameters are given by García de la Rosa (1987) and Muller (1992).

Considering a model in which the sunspot consists of a single monolithic magnetic column, Knobloch & Weiss (1984) showed that in a nonlinear treatment of magnetoconvection vertical energy transport is occuring inside the rigid magnetic field structure. In their picture UDs are the visible consequence of coherent motions in convective cells with diameters of 250-300 km reaching down to a depth of [FORMULA] 1500 km.

The so-called cluster model, proposed by Parker (1979) and elaborated by Choudhuri (1986), proceeds from the assumption that the sunspot magnetic field splits below the visible umbral photosphere in several individual bundles of flux tubes separated by nearly fieldfree solar plasma. Overstable convection can occur in these fieldfree regions and UDs are believed to be the manifestation for hot columns of gas, rising up between the magnetic ropes.

In a more recent theoretical study, Degenhardt & Lites (1993) investigate the behaviour of a thin vertical gas column embedded in a sunspot umbra using the thin flux tube approximation (Ferriz-Maz & Schüssler 1990).

The magnetic field strength in UDs should be greatly reduced at the visible surface and accompanied by substantial upflows ([FORMULA]  10 km/s) if we believe the Parker/Choudhuri picture. The observable signatures in the model of Knobloch & Weiss would be large fluctuations in field strength due to the highly nonlinear oscillations, while Degenhardt & Lites predict only small velocity and magnetic field differencies in photospheric layers and an inverse field gradient in UDs.

However, these model characteristics are not in accord with existing observations. On the other hand observations of UDs are strongly influenced by the spatial resolution of the instrument and seeing conditions. Thus from the observational point of view it is as well difficult to rule out or confirm any of the models.

The brightness and temperature of UDs has been studied by various authors (e.g. Beckers & Schröter 1968; Koutchmy & Adjabshirzadeh 1981; Grossmann-Doerth et al. 1986; Sobotka et al. 1992a, 1992b; Ewell 1992; Sobotka et al. 1993) who agree that the wide range of measured UD contrasts and derived temperatures points to the existence of more than one kind of UDs, differing in their individual intrinsic development.

Spectroscopic investigations allow to infer the magnetic field strength inside umbrae. In observations of moderate spatial resolution ([FORMULA]), UDs tend to show a slightly weaker field strength than the surrounding umbra. The claimed reduction in field strength varies between some 100 Gauss (Beckers & Schröter 1969; Adjabshirzadeh & Koutchmy 1983; Pahlke & Wiehr 1990) up to nearly 50 % (Kneer 1973).

Observations of higher spatial resolution show more contradicting results. Lites et al. (1991) find that UDs are not accompanied by significant variations of magnetic field strength at the visible surface. We refer to Grossmann-Doerth et al. (1986) and distinguish between central and peripheral UDs (CUDs and PUDs respectively). Schmidt & Balthasar (1994) find a field reduction in CUDs of [FORMULA] 10-20 % and in PUDs of [FORMULA] 5-10 %. In a more recent study Balthasar & Schmidt (1994) report about a decrease of magnetic field strength in bright umbral structures by 5-10 % for an infrared line. A weakening of the field strength of [FORMULA] 20 % in PUDs and almost none in CUDs has been reported by Wiehr & Degenhardt (1993).

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998