Persistent Coronal Streamers and the Identification of Sunspot Clusters

Abstract

We use limb synoptic plots to study long-lived features of the lower solar corona. The most persistent features are the polar sinusoids, which are generated by streamers associated with active regions. We find that the lifetimes of these structures (up to about 10 solar rotations) are much longer than the lifetimes of individual sunspots (typically less than one solar rotation). The long lifetimes of the polar sinusoids are due to clusters of spatially related but noncontemporaneous spots. The continuous emergence of sunspots and magnetic flux from spot clusters in the photosphere provides the long life spans of the coronal streamers. Two-thirds of the ≈180 sunspots recorded in the southern hemisphere in a 1-year period near the 1996-1997 solar minimum were members of noncontemporaneous clusters. The clusters suggest large-scale, long-lived structures in the subphotospheric magnetic field from which sunspots emerge.