Magnetic reconnection in astrophysical plasmas

Abstract

This thesis addresses the problem of understanding magnetic reconnection in astrophysical plasmas. After reviewing earlier work, we present results from the numerical simulation of two periodic reconnection models. Our simulations show that reconnection stalls as the magnetic field at the onset of the current sheet cannot exceed the background magnetic field. Our numerical results imply that analytical solutions are required for gaining a better understanding of reconnection. First we address the steady-state problem by making a sub-Alfvénic approximation but, unlike previous approaches, we do not linearise the resulting equations. Our solutions show that magnetic field lines can only be annihilated under the sub--Alfvénic approximation - reconnection cannot take place. Next we derive exact solutions of steady-state reconnection by combining our sub-Alfvénic findings with solutions for the annihilation of anti-parallel magnetic fields. The key point of this approach was to relax the traditional inner symmetries for the magnetic field and plasma flow. Our solutions strongly suggest that steady-state reconnection requires a lack of topological symmetry. Finally our exact steady-state solutions are extended to include time-dependency. Here the current density at the neutral point attains two maximums and we demonstrate that only one maximum gives a significant energy release. It is also outlined how our exact steady-state and time-dependent solutions can be extended to three dimensions.