Cold ion energization at separatrices during magnetic reconnection

Abstract : Separatrices of magnetic reconnection host intense perpendicular Hall electric fields. The fields are produced by the decoupling of the ion and electron components and are associated with the in-plane electrostatic potential drop between the inflow and outflow regions. The width of these structures is typically less than the ion inertial length, which is small enough to demagnetize ions as they cross the layer. We investigate ion acceleration at separatrices by means of 2D particle-in-cell simulations of magnetic reconnection for two limiting cases: (1) a “GEM-like” setup (here GEM stands for geospace environmental modeling reconnection challenge) with the lobe ion thermal velocity equal to the thermal velocity of the initial current sheet ions, which is comparable to the Alfvén velocity and (2) a “cold” ion setup, in which the temperature of the background lobe ions is 1/100 of the initial current sheet temperature. The separatrix Hall electric field is balanced by the ion inertia term in the cold background simulations. The effect is indicative of the quasi-steady local perpendicular acceleration. The electric field introduces a cross field beam of unmagnetized particles, which makes the ion distribution function strongly non-gyrotropic and susceptible to sub-ion scale instabilities. This acceleration mechanism nearly vanishes in the hot ion background simulations. Our particle-in-cell simulations are complemented by one-dimensional test particle calculations. They show that the hot ion particles experience energy-scattering after crossing the accelerating layer, whereas cold ions are uniformly energized up to the energies comparable to the electrostatic potential drop between the inflow and outflow regions.