On the Ion Distributions at the Separatrices during Symmetric Magnetic Reconnection

A particle-in-cell simulation of symmetric reconnection with zero guide field is carried out to understand the dynamics of ions along separatrices. Through the investigation of spatial and temporal evolution of ion velocity distributions, a typical distribution along the separatrices is found: two counter-streaming cores with a parallel direction core population. Backward tracing of ions reveals that the counter-streaming cores are mostly composed of ions initially located at the same side of the separatrix, while the parallel direction core population is composed of ions originally located on the opposite side of the separatrix, crossing through the neutral sheet. Through the analysis of energy conversion, it is found that along the separatrix, the ion energization is mainly contributed by the Hall electric field, and the reconnection electric field dominates the ion energization in the region between two separatrices. The ions in the low-energy part of the counter-streaming population cross the separatrix twice, affected by the combination of Hall electric field and reconnection electric field. The ions in the high-energy part directly enter the separatrix from the unperturbed plasma, energized mainly by the Hall electric field. The population in the parallel direction is firstly accelerated by the Hall electric field on the opposite side of the separatrix, then is decelerated by the Hall electric field, and finally is accelerated by the in-plane electric field and reconnection field.