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 line which are tangentially touched by coronal field lines. These locations are called 'Bald Patches'. Field lines threading these bald patch regions, together with those threading any coronal null points, determine the 'Topological Skeleton' of the field. We examine the properties of these null points and bald patch regions for two, three and four magnetic sources, and study the evolution of the coronal topological skeleton as a result of moving and varying the sources. In particular, for a four-source magnetic configuration, we establish the existence of a well-defined 'separator' field line in the absence of any coronal null points. We conclude that bald patches as well as coronal nulls constitute an elemental topological feature of the coronal field: they may be significant as possible regions for coronal current sheet formation and for prominence formation.