The topological behaviour of 3D null points in the Sun's corona

Many traditional two-dimensional models of flares and other reconnective events rely on an X-point in the corona which collapses to form a current sheet where reconnection takes place. However, the Sun's corona is three-dimensional and the X-point is only a two-dimensional artifact. Three-dimensional models can instead make use of 3D null points in the corona, and Priest & Titov (1996) have studied how reconnection may take place at such locations. This paper aims to study possible conditions for coronal nulls to occur, and how they can be born. It starts with three negative sources and a positive source that is weaker than the sum of the negative sources. As the positive source moves inwards towards the negative sources, there is a local double-separator bifurcation of the topology to a new state, referred to as a Coronal Null state, which contains a null point above the photosphere.