3.1 Observations of umbral dots

Sunspots umbrae show about 20%3 of the quiet Sun luminosity, a value which cannot entirely be explained through radiative energy transport. Indeed, observations with a spatial resolution better than 0.5 arcsec reveal that the umbra harbors dynamic inhomogeneities, which have been called umbral dots by Danielson (1964). They are observed as bright dot-like spots with typical sizes of half an arcsec or less, embedded in a more uniform and darker background. These umbral dots seem to be present in all sunspots, although their intensity varies significantly. In some spots they can be almost as bright as bright penumbral filaments, in other spots their intensity is much smaller. In the latter case, the dot-like intensity variations occurs in a background that shows a lower intensity.

Umbral dots are an obvious signature of convection, yet it is not so obvious to understand the type of convection that leads to umbral dots. In the field-free gap idea of Parker, the convection is confined by the strong surrounding magnetic field, such that the column of convection narrows upwards and only a small brightening is seen at the surface. It is established observationally that the magnetic field in umbral dots is weaker than in the surroundings and that an upflow of at least a few hundred m s–1 is associated with them (Socas-Navarro et al., 2004; Rimmele, 2004, 2008Jump To The Next Citation Point; Bharti et al., 2007). The latter two observations also establish the presence of dark lanes across umbral dots. From observations it is still unclear how small the field strength in umbral dots really is, since it is difficult to resolve the field strength gradients along the LOS and because there are stray light issues. Numerical models presented in Section 3.6 point toward a dramatic reduction to almost zero field strength in sub-photospheric layers, however, the reduction in line forming regions is less pronounced.

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