Magnetic Reconnection: 3D Magnetic Null Points

Three dimensional magnetic null points are now accepted as important topological features at which magnetic reconnection occurs. However, the understanding of the processes involved is still far behind the well developed field of 2D X-point reconnection. Therefore, the aim of this thesis is to present realistic extensions of the known ways in which 3D null point reconnection occurs. The Torsional (twisting) regimes of 3D null point reconnection are investigated using analytical models with, for the first time, localised current structures that qualitatively match those seen in simulation studies. These solutions show a wealth of possible scenarios in which new connections can form as a result of twisting perturbations near 3D nulls.
Analytical solutions for fan and spine reconnection are presented with asymmetric current sheets as this scenario is thought to be commonplace in astrophysical plasmas. The asymmetry in each solution has a profound and rather different effect in each case. This analysis is then complimented by a series of numerical experiments studying the self consistent formation of similar current structures for the spine-fan mode in response to transient driving. Time dependent effects, such as the movement of the null position and the applicability of scaling laws derived from analyses with symmetric current sheets, are discussed. These results suggest that, in typical astrophysical plasmas, 3D null points may be continuously shifting position with a flow of plasma at the null point itself. Lastly, as instabilities are thought to play an important role in astrophysical reconnection dynamics, a series of numerical experiments investigating the self consistent formation and subsequent instability of a current-vortex layer at the fan plane of a 3D null point is presented. The results suggest that separatrix surfaces are great potential sites for current-vortex sheet formation and, therefore, the additional heating and connection change associated with instabilities of this layer.