Electron kinetics in collisionless magnetic reconnection

Abstract

In nearly collisionless astrophysical plasmas and magnetic fusion experiments it is of interest to understand the possibilities of fast magnetic reconnection processes. Fluid modelling of collisionless magnetic reconnection in the presence of a strong magnetic guide field has demonstrated high reconnection rates where electron inertia and electron compressibility play an important role. Numerical simulations show that the high reconnection rate involves the acceleration of electrons in an exceedingly thin current layer. However, in this fluid model of reconnection, smaller and smaller length scales develop in the current density and vorticity distributions. More appropriate for a collisionless plasma is a kinetic model. Beside the fact that kinetic effects offer additional reconnection mechanisms, kinetic effects also modify the reconnection process due to electron inertia. Kinetic effects such as thermal broadening resolve the singularities that arise in fluid models while allowing for a fast reconnection rate. In addition, kinetic theory offers an interpretation of the current structures that form in terms of a resonance with thermal electrons. Another point of interest is that reconnection in a magnetically confined plasma can bring together field lines with a temperature or density difference. A kinetic description resolves the steep gradients that would arise in a collisionless fluid model. This paper analyses these issues on the basis of stationary reconnection processes. Stationarity rules out the additional kinetic effects of wave–particle interactions such as Landau resonances.