Context. The magnetic field in many astrophysical plasmas,for example in the solar corona, is known to have a highly complex -and clearly three-dimensional - structure. Turbulent plasma motions inhigh-regions where field lines are anchored, such as the solar interior, canstore large amounts of energy in the magnetic field. This energy canonly be released when magnetic reconnection occurs. Reconnection mayonly occur in locations where huge gradients of the magnetic fielddevelop, and one candidate for such locations are magnetic null points,known to be abundant for example in the solar atmosphere. Reconnectionleads to changes in the topology of the magnetic field, and energybeing released as heat, kinetic energy and acceleration of particles.Thus reconnection is responsible for many dynamic processes, forinstance flares and jets. Aims. The aim of this paper is to investigate the properties ofmagnetic reconnection at a 3D null point, with respect to theirdependence on the symmetry of the magnetic field around the null. Inparticular we examine the rate of reconnection of magnetic flux at thenull point, as well as how the current sheet forms and its properties. Methods. We use mathematical modelling and finite difference resistive MHD simulations. Results. It is found that the basic structure of the mode ofmagnetic reconnection considered is unaffected by varying the magneticfield symmetry, that is, the plasma flow is found to cross both thespine and fan of the null. However, the peak intensity and dimensionsof the current sheet are dependent on the symmetry/asymmetry of thefield lines. As a result, the reconnection rate is also found to bestrongly dependent on the field asymmetry. Conclusions. The symmetry/asymmetry of the magnetic field in thevicinity of a magnetic null can have a profound effect on the geometryof any associated reconnection region, and the rate at which thereconnection process proceeds. Key words: magnetohydrodynamics (MHD) - magnetic reconnection - Sun: corona - Sun: magnetic topology
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