Solar active regions: a nonparametric statistical analysis

Context. The sunspots and other solar activity indicators tend to cluster on the surface of the Sun. These clusters very often occur at certain longitudes that persist in time. It is of general interest to find new and simple ways to characterize the observed distributions of different indicators and their behaviour in time. Aims. In the present work we use Greenwich sunspot data to evaluate the statistical but not totally coherent stability of the sunspot distribution along latitudes as well as longitudes. The aim was to obtain information on the longitudinal distribution of the underlying spot-generating mechanism rather than on the distribution and migration of sunspots or sunspot groups on the solar surface. Therefore only sunspot groups were included in the analysis, and only the time of their first appearance was used. Methods. We used a simple nonparametric approach to reveal sunspot migration patterns and their persistency. Results. Our analysis shows that regions where spots are generated tend to rotate differentially as the spots and spot groups themselves do. The spatial correlations in activity, however, tend to break down relatively fast, during 7–15 solar rotations. Conclusions. This study provides a challenge for solar dynamo models, as our results indicate that the non-axisymmetric spot-generating mechanism experiences differential rotation (known as phase mixing in dynamo theory). The new nonparametric method introduced here, completely independent of the choice of the longitudinal distribution of sunspots, was found to be a useful tool for spatio-temporal analysis of surface features