BASIC TOPOLOGICAL ELEMENTS OF CORONAL MAGNETIC-FIELDS

We model the magnetic field of a coronal active region using discrete magnetic sources and look for regions of the photospheric inversion li ne which are tangentially touched by coronal field lines. These locati ons are called 'Bald Patches'. Field lines threading these bald patch regions, together with those threading any coronal null points, determ ine the 'Topological Skeleton' of the field. We examine the properties of these null points and bald patch regions for two, three and four m agnetic sources, and study the evolution of the coronal topological sk eleton as a result of moving and varying the sources. In particular, f or a four-source magnetic configuration, we establish the existence of a well-defined 'separator' field line in the absence of any coronal n ull points. We conclude that bald patches as well as coronal nulls con stitute an elemental topological feature of the coronal field: they ma y be significant as possible regions for coronal current sheet formati on and for prominence formation.