Cold ion effects in density-asymmetric collisionless magnetic reconnection

The coexistence of low-energy (cold) ions and thermal (warm) ions is commonly observed in space and laboratory plasmas, such as those in magnetopause and fusion fueling processes. In certain events, the cold ion proportion may play a crucial role in plasma processes, especially magnetic reconnection. In this paper, magnetic reconnection with density-asymmetric cold ions is investigated in implicit particle-in-cell (iPIC) simulations. It is found that in such events the reconnection rate decreases as the cold ion distribution depth into the current sheet increases, mainly due to the mass-loading effect. Particularly, a density-peak structure of cold ions is developed in the reconnection region owing to the bounce motion of cold ions entering from the opposite inflow region. In the y-vy phase space where the y-direction is normal to the current sheet, a cold ion ring structure related to the bounce motion is formed and amplified by the Hall electric field. Furthermore, the cold ions become a notable current carrier due to its shorter inertial scale than the warm ions. Consequently, the asymmetry of the cold ion distribution significantly breaks the symmetry in the Hall magnetic field, eventually leading to asymmetric cold ion density peak structure. Such structures can be taken as significant signals of cold ion existence in in situ spacecraft observations.