A Modified Multi-Level Tracking Scheme for the Detection of Sunspot Umbral Dots

Abstract

Umbral dots (UDs) are small-scale convective intrusions in the umbral core of sunspots and pores. Different methods have been used in the past to determine the physical properties of UDs. One of the methods typically used is multi-level tracking (MLT), which tags spatial structures at equi-spaced intensity levels from the highest level while progressing downward. A modified approach to the regular MLT is explored in this article that also uses the local intensity maxima with a change in the threshold condition to enclose a UD, such that diffuse UDs do not appear extended than they visually appear. The physical properties of UDs from these two MLT approaches are compared. The methods are implemented on high-resolution blue continuum images of four sunspots from the 50-cm Solar Optical Telescope on board Hinode. In addition, we introduce a density-based, spatial clustering routine for the first time to ascertain the differences resulting from the two tracking methods. The modified MLT approach yields an effective diameter with median values ranging from 250-310 km which is on average 70-90 km smaller than the regular MLT approach. The lower effective diameter in the modified method is associated with a reduced UD fill fraction of 12%-13% while the regular method yields 17-19%. However, these differences are still within the range of values cited by earlier works. On the other hand, the histogram of the mean intensity of UDs from both methods is nearly identical. The spatial clustering of UDs from both methods also shows very similar results. There is, however, a preferential spatial concentration of UDs, particularly at locations where the umbral core is highly irregular and in the vicinity of faint light bridges. The dependency of the localized clustering of UDs on the evolutionary phase of the sunspot and its magnetic complexity needs to be further explored.