Cold Ion Behavior during Asymmetric Magnetic Reconnection at the Magnetopause

What is the controlling factor of asymmetric magnetic reconnection and what is the role of cold plasma population in the reconnection process are two outstanding questions. In the present study, we have found that the separatrix angles and the flow boundary angles on the magnetosheath side are larger than those on the magnetospheric side. This may be caused by the stronger asymmetry in the plasma density than that in the magnetic field. We also find that the motion of the cold ion from the plasmasphere is different from that of the transmitted magnetosheath ion and thermal magnetospheric ion. The plasmaspheric ions have less speed than that of the transmitted magnetosheath ions and thermal magnetospheric ions, which are probably accelerated by the reconnection process. The cold dense plasmaspheric ions are suggested to be picked up by the recently reconnected magnetic field at high latitude rather than in the diffusion region near the subsolar magnetopause. These cold ions are accelerated by the electric field and carried by the reconnection convection flow which is similar with the pickup ion process. This indicates that the plasmaspheric ions play a very limited role in the asymmetric magnetic reconnection process, in contrast to previous simulation studies. It is also evident that three cold ion populations (probably H+, He+, and O+) appear in the energy spectrum once they are accelerated to the same velocity since the observed energy of He+ and O+ ions are 4 and 16 times higher than that of H+, respectively. For all 28 reconnection events in the presence of cold ions, cold ions are accelerated perpendicular to the magnetic field. In addition, cold ions are not observed on the magnetosheath side of the reconnection region except for one unclear event. For three events, different cold ion populations (probably, H+, He+, and O+) appear in the energy spectrum. These events occur when Kp index and solar wind speed are high.