A New Look at the Electron Diffusion Region in Asymmetric Magnetic Reconnection. Issue 2 (28th January 2021) - Hesse, Michael
Abstract: A new look at the structure of the electron diffusion region in collision less magnetic reconnection is presented. The research is based on a particle‐in‐cell simulation of asymmetric magnetic reconnection, which includes a temperature gradient across the current layer in addition to density and magnetic field gradient. We find that none of X‐point, flow stagnation point, and local current density peak coincide. Current and energy balance analyses around the flow stagnation point and current density peak show consistently that current dissipation is associated with the divergence of nongyrotropic electron pressure. Furthermore, the same pressure terms, when combined with shear‐type gradients of the electron flow velocity, also serve to maintain local thermal energy against convective losses. These effects are similar to those found also in symmetric magnetic reconnection. In addition, we find here significant effects related to the convection of current, which we can relate to a generalized diamagnetic drift by the nongyrotropic pressure divergence. Therefore, only part of the pressure force serves to dissipate the current density. However, the prior conclusion that the role of the reconnection electric field is to maintain the current density, which was obtained for a symmetric system, applies here as well. Finally, we discuss related features of electron distribution function in the electron diffusion region (EDR). Specifically, we analyze both new crescent substructures as well as outer, higher energy crescents generated by accelerated magnetospheric particles. Plain Language Summary: Magnetic reconnection is arguably the most important mechanism to release energy stored in magnetic fields explosively. Magnetic reconnection is believed to be the driver between as diverse a set of phenomena as solar eruptions, astrophysical radiation bursts, magnetic storms in near‐Earth space, and the aurora. Quite amazingly, magnetic reconnection facilitates energy conversion over huge regions of space with size of many Earth radii by means of a tiny core region, the so‐called diffusion region, with dimensions of a few to a few hundreds of kilometers. The delicate interaction between charged particles and electromagnetic fields in this central region enables the large‐scale conversion of magnetic energy to particle energy to proceed. This paper presents a new look at the inner workings of this region for a fairly generic case of magnetic reconnection, which, among others, occurs at the interface between the Earth's magnetic field and the particle streams originating at the Sun. Key Points: Flow stagnation point and current density maximum are not necessarily collocated in asymmetric magnetic reconnection The reconnection electric field sustains the electron current density and pressure also in asymmetric magnetic reconnection Electron crescent distributions feature complex substructures related to different electron inflow regions