On the size distribution of spots within sunspot groups

Abstract

The size distribution of sunspots provides key information about the generation and emergence processes of the solar magnetic field. Previous studies of size distribution have primarily focused on either the whole group or individual spot areas. In this paper we investigate the organisation of spot areas within sunspot groups. In particular, we analysed the ratio (R) of the area of the biggest spot (Abig_spot) inside a group, to the total area of that group (Agroup). We used sunspot observations from Kislovodsk, Pulkovo, and Debrecen observatories, together covering solar cycles 17–24. We find that at the time when the group area reaches its maximum, the single biggest spot in a group typically occupies about 60% of the group area. For half of all groups, R lies in the range between roughly 50% and 70%. We also find R to change with Agroup, such that R reaches a maximum of about 0.65 for groups with Agroup ≈ 200 μHem and then remains at about 0.6 for larger groups. Our findings imply a scale-invariant emergence pattern, providing an observational constraint on the emergence process. Furthermore, extrapolation of our results to larger sunspot groups may have a bearing on the giant unresolved starspot features found in Doppler images of highly active Sun-like stars. Our results suggest that such giant features are composed of multiple spots, with the largest spot occupying roughly 55–75% of the total group area (i.e., the area of the giant starspots seen in Doppler images).