Turbulent transport in 2D collisionless guide field reconnection

Abstract

Transport in hot and dilute, i.e., collisionless, astrophysical and space, plasmas is called “anomalous.” This transport is due to the interaction between the particles and the self-generated turbulence by their collective interactions. The anomalous transport has very different and not well known properties compared to the transport due to binary collisions, dominant in colder and denser plasmas. Because of its relevance for astrophysical and space plasmas, we explore the excitation of turbulence in current sheets prone to component- or guide-field reconnection, a process not well understood yet. This configuration is typical for stellar coronae, and it is created in the laboratory for which a 2.5D geometry applies. In our analysis, in addition to the immediate vicinity of the X-line, we also include regions outside and near the separatrices. We analyze the anomalous transport properties by using 2.5D Particle-in-Cell code simulations. We split off the mean slow variation (in contrast to the fast turbulent fluctuations) of the macroscopic observables and determine the main transport terms of the generalized Ohm's law. We verify our findings by comparing with the independently determined slowing-down rate of the macroscopic currents (due to a net momentum transfer from particles to waves) and with the transport terms obtained by the first order correlations of the turbulent fluctuations. We find that the turbulence is most intense in the “low density” separatrix region of guide-field reconnection. It is excited by streaming instabilities, is mainly electrostatic and “patchy” in space, and so is the associated anomalous transport. Parts of the energy exchange between turbulence and particles are reversible and quasi-periodic. The remaining irreversible anomalous resistivity can be parametrized by an effective collision rate ranging from the local ion-cyclotron to the lower-hybrid frequency. The contributions to the parallel and the perpendicular (to the magnetic field) components of the slowly varying DC-electric fields, balanced by the turbulence, are similar. This anomalous electric field is, however, smaller than the contributions of the off-diagonal pressure and electron inertia terms of Ohm's law. This result can now be verified by in-situ measurements of the turbulence, in and around the magnetic reconnection regions of the Earth's magnetosphere by the multi-spacecraft mission MMS and in laboratory experiments like MRX and VINETA-II.