Magnetic reconnection at 3D null points: effect of magnetic field asymmetry

Division of Mathematics, University of Dundee, UK e-mail: akhalhachami@maths.dundee.ac.uk

Received: 28 July 2009
Accepted: 23 January 2010

Abstract

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 in high-β regions where field lines are anchored, such as the solar interior, can store large amounts of energy in the magnetic field. This energy can only be released when magnetic reconnection occurs. Reconnection may only occur in locations where huge gradients of the magnetic field develop, and one candidate for such locations are magnetic null points, known to be abundant for example in the solar atmosphere. Reconnection leads to changes in the topology of the magnetic field, and energy being released as heat, kinetic energy and acceleration of particles. Thus reconnection is responsible for many dynamic processes, for instance flares and jets.

Aims. The aim of this paper is to investigate the properties of magnetic reconnection at a 3D null point, with respect to their dependence on the symmetry of the magnetic field around the null. In particular we examine the rate of reconnection of magnetic flux at the null 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 of magnetic reconnection considered is unaffected by varying the magnetic field symmetry, that is, the plasma flow is found to cross both the spine and fan of the null. However, the peak intensity and dimensions of the current sheet are dependent on the symmetry/asymmetry of the field lines. As a result, the reconnection rate is also found to be strongly dependent on the field asymmetry.

Conclusions. The symmetry/asymmetry of the magnetic field in the vicinity of a magnetic null can have a profound effect on the geometry of any associated reconnection region, and the rate at which the reconnection process proceeds.