Internal structure of sunspot umbrae

On the basis of high resolution and low scattered light photographs of different sunspots and their umbrae the behaviour of umbral structural details was investigated. The main results are as follows: each sunspot umbra, regardles of its size, magnetic field intensity, darkness, form, age and presence of a penumbra, has an identical internal morphological structure, formed of a dark network of cellular elements, the center of each being filled with a bright grain. Morphologically this structure does not differ from the network of photospheric granules, only the apparent width of the dark intergranular space grows and its darkness deepens with the increasing intensity of the local magnetic field. The constructed structural maps representing the positions of the bright central parts of the cellular elements, are completely identical for both photospheric as well as for umbral granular fields, they show the same surface density of the individual bright element central positions and the same characteristic length scale of bright element distribution of 1.2'' to 1.4''. The character of the internal umbral structure does not change during the whole process of spot development and does not depend on the phase of the solar activity cycle either. In the structural maps, it is impossible to find any indication of sunspot existence. The life-time of the individual umbral cells seems to fall within the interval of 10 to 30 minutes, although certain cellular elements live much longer. During the study, the fundamental physical importance of the dark intergranular space in the photosphere was again emphasized. The question of a more general relationship between the special behaviour of the dark cool intergranular plasma with an imbedded magnetic field was discussed. (author)