We turn now to a discussion of the fine structure of a sunspot, beginning here with features in the umbra and continuing in the next chapter with features in the penumbra. Our knowledge of this fine structure has been transformed in recent years due to remarkable improvements in high-resolution observations. We review the results of these observations and theoretical interpretations of them. Our enhanced knowledge of the fine structure of sunspots has not only provided us with a far larger collection of details; it has also stimulated new insights that allow us to start assembling a coherent picture of the formation and maintenance of a sunspot, with its dark umbra and its puzzling filamentary penumbra.
In many images, sunspot umbrae – like pores, which are just isolated umbrae – appear uniformly dark. When such images are appropriately exposed, however, as in Figure 4.1, it becomes apparent that there is an intensity pattern in sunspot umbrae, composed of many small, isolated, bright features embedded in a darker, smoothly varying background. These features are called umbral dots and they are found in essentially all sunspots and also in pores (Sobotka 1997, 2002). Earlier observations of an intensity pattern in umbrae, with a resolution of about 1″, had failed to resolve the umbral dots and instead showed a pattern that looked more like a weaker version of the photospheric granulation (Chevalier 1916; Bray and Loughhead 1964).