Acceleration and Phase‐Space Structure Formation of Cold Ions in Collisionless Reconnection

Abstract Particle acceleration and the formation of characteristic phase space structures far from thermal equilibria along with their underlying mechanisms constitute key research topics across multiple disciplines. These phenomena hold particular significance in plasma physics and span applications from fundamental physics to space science and technology. Magnetic reconnection involving cold ions is ubiquitous in both space plasmas and laboratory fusion plasmas. Compared to other particle populations, cold ions exhibit more distinct kinetic signatures of acceleration and non‐equilibrium dynamics. In this work, we analyze typical processes for collisionless reconnection with cold ions in an initial two‐dimensional Harris sheet equilibrium by numerical simulation. It is found that the formation of the phase‐space structure of the cold ions is closely related to the topological property of magnetic configurations. For those entering the diffusion region around the X‐point, cold ions are accelerated mainly by the reconnection electric field, and a clump‐like structure in the phase‐space is formed near the outflow region. On the other hand, for those entering the exhaust region across the separatrix away from the X‐point, cold ions are accelerated mainly by the Hall electric field. In this case, gyro‐phase‐dependent acceleration produces a crescent‐shaped velocity‐space distribution that represents the visible projection of an underlying spiral phase‐space structure within the separatrix. Acceleration processes and formation mechanisms of phase‐space structures are further analyzed using the particle tracing method. These results highlight the critical role of magnetic topology and localized electric fields in governing cold‐ion acceleration and nonequilibrium phase‐space structuring during collisionless magnetic reconnection.