Observations of umbral dots and their physical models

Abstract

Abstract The Hinode satellite opens a new era in sunspot research, because of its high spatial resolution and temporal stability. Fine-scale structures in sunspots, called umbral dots (UDs), have become one of the hottest topics in terms of close observations of magnetoconvection. In this paper, a brief review of the observed properties of UDs is given based on recent literature. UDs born in the periphery of the umbra exhibit inward migration, and their speeds are positively correlated with the magnetic field inclination. Longer-lasting UDs tend to be larger and brighter, while the lifetimes of UDs show no relation to their background magnetic field strength. UDs tend to disappear, or stop their proper motion by colliding with a locally strong field region. The spatial distribution of UDs is not uniform over an umbra, but is rather located at the boundaries of cellular patterns. From our two-dimensional correlation analysis, we measured the characteristic width of the cell boundaries (≈ 0${^{\prime\prime}_{.}}$5) and the size of the cells (≈ 6″). We then performed a simplified analysis to obtain statistics of how the UD distribution is random or clustered using Hinode blue continuum images. We have found a hint that the UDs become less dense and more clustered for later-phase sunspots. These results may be related to the evolutional change of the subsurface structure of a sunspot. Based on these observational results, we discuss their physical models by means of numerical simulations of magnetoconvection.