Sunspot nests as traced by a cluster analysis
The appearance of sunspot groups between August 1959 and December 1964 is investigated in search of sunspot nests. A sunspot nest is a relatively small space on the surface within which a succession of spot groups appears. A single-linkage clustering technique is used to trace clusters in the three dimensions longitude, latitude and time. The number of genuine sunspot nests is estimated and their properties are determined.
At least one third of the sunspot appearances belong to intrinsically physical clusters. Even a substantial fraction of the clusters of two-member groups is genuine.
The clustering criteria are set such that the smallest scales in the clustering are brought out. The sizes of the nests turn out very small: the typical (1σ-) spread about the longitude-time regression line is about 2°, and the spread about the mean latitude is about 1°. The lifetimes range from one to seven months. The nests follow the rotation rate-latitude relation of recurrent sunspots within a small but intrinsic spread of about 15 m s−1. In the present sample the displacements in latitude are extremely small: the amplitude of any large-scale flow pattern must be less than 3 m s−1, whereas the intrinsic proper motions in latitude are less than 5 m s−1.
The appearance of spot groups in a nest is intermittent. On one hand, in an active nest spots may be absent for nearly two complete rotations. On the other hand, in many nests more than one spot group has been recorded during one disk crossing.
There is a strong tendency for the nests to cluster once again in larger nests: more than 1/3 of the compact nests are components in larger nests. Within such a large nest the components overlap in time, their mean latitudes differ by less than 2°.5, but the difference in longitude may extend up to 55°.
The present and earlier results are either in harmony, or seemingly discordant results are readily explained by properties of the nests and differences in the search criteria.
KeywordsCluster Technique Proper Motion Sunspot Group Search Criterion Discordant Result
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