Sunspot photometry with phase diversity - II. Fine-structure characteristics

We investigate the thermal and morphological fine structure of a small sunspot, which includes the determination of brightness temperatures and characteristic spatial scales as well as their distribution inside the sunspot. The identification and isolation of sunspot fine structure is accomplished by means of a feature-finding algorithm applied to a high-resolution time sequence taken simultaneously in three continuum bands of the solar spectrum. In order to compensate for seeing and instrumental effects, we apply the phase-diversity technique combined with a deconvolution method. The findings can be summarized as follows: (1) umbral dots are found to be on average 760 K cooler than the immediate surrounding photosphere outside the spot. (2) Some exceptional hot penumbral grains exceed the average temperature of the brightest granules of the spots surroundings by typically 150 K. (3) The size distribution of umbral dots and penumbral grains support the idea that the smallest structures are still spatially unresolved. (4) The distribution function of umbral dot peak intensities points to the existence of two umbral dot populations indicating different efficiency of energy transport. (5) The classification of penumbral filaments into dark and bright depends on the immediate surroundings.