Relativistic magnetic reconnection in collisionless ion-electron plasmas explored with particle-in-cell simulations

Magnetic reconnection is a leading mechanism for magnetic energy conversion and high-energy non-thermal particle production in a variety of high-energy astrophysical objects, including ones with relativistic ion-electron plasmas (e. g., microquasars or AGNs), a regime where first principle studies are scarce. We present 2D particle-in-cell (PIC) simulations of low beta ion-electron plasmas under relativistic conditions, i.e., with inflow magnetic energy exceeding the plasma restmass energy. We identify outstanding properties: (i) For relativistic inflow magnetizations (here 10 <= sigma(e) <= 360), the reconnection outflows are dominated by thermal agitation instead of bulk kinetic energy. (ii) At high inflow electron magnetization (sigma(e) >= 80), the reconnection electric field is sustained more by bulk inertia than by thermal inertia. It challenges the thermal-inertia paradigm and its implications. (iii) The inflows feature sharp transitions at the entrance of the diffusion zones. These are not shocks but results from particle ballistic motions, all bouncing at the same location, provided that the thermal velocity in the inflow is far lower than the inflow E x B bulk velocity. (iv) Island centers are magnetically isolated from the rest of the flow and can present a density depletion at their center. (v) The reconnection rates are slightly higher than in non-relativistic studies. They are best normalized by the inflow relativistic Alfven speed projected in the outflow direction, which then leads to rates in a close range (0.14-0.25), thus allowing for an easy estimation of the reconnection electric field.