Phase-space Structures of Cold Ions in the Outflow Region of Collisionless Reconnection

Cold ions of multi-origins are ubiquitous in the Earth's magnetosphere and significantly influence the dynamics of magnetic reconnection. Due to their low temperature and small gyroradius, cold ions exhibit kinetic features distinct from those of warm ions. In this study, we investigate the non-thermal equilibrium phase-space structures and acceleration mechanisms of cold ions in the reconnection outflow region using 2.5D particle-in-cell (PIC) simulations. We identify three distinct regions characterized by unique cold-ion velocity-space distributions. Near the X-line, cold ions accelerated by the reconnection electric field (Ez) exhibit a counter-streaming bounce motion, forming discrete beam-like structures. In the mid-outflow region, upstream of the dipolarization front, cold ions form "rabbit-ear" or “\pi-shaped” distributions. Particle tracing reveals that these structures arise from the combined effects of the Hall electric field and the specific crossing order of ions across the separatrix. This process also generates super-Alfvénic ion beams via Lorentz force. Inside the magnetic island, cold ions undergo secondary acceleration by the enhanced Ez, forming a "V-shaped" distribution with high bulk velocities. We further demonstrate that the macroscopic "stripe-like" density patterns observed in the outflow are a manifestation of these kinetic phase-space evolutions. These findings provide crucial kinetic diagnostics for identifying spacecraft locations relative to the reconnection X-line based on cold-ion signatures.