We summarize our recent studies of large-scale, long-lived coronal structures detected in limb synoptic maps using Yohkoh/SXT data. These structures appear as "polar sinusoids" on full limb synoptic maps made from data. A complete limb synoptic map is displayed, which covers from October 1991 to August 2001 (almost the entire Yohkoh mission). Several properties of the coronal polar sinusoids have been revealed. Comparison with optical data shows that these vast coronal structures are associated with underlying, non-contemporaneous sunspot clusters. Individual spots in the clusters are short-lived, but contribute magnetic flux collectively to sustain the long-lived coronal features. This implies the existence of a subphotospheric magnetic region from which the sunspots erupt.
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