Energization of Cold Ions in Magnetic Reconnection: Particle-in-Cell Simulation

Cold ions in the ionosphere can reach the nightside magnetotail along the magnetic field lines, which may substantially affect the magnetotail reconnection and reconnection-driven phenomena. Although reconnection has been intensively studied in the past decades, the dynamics of cold ions in reconnection are poorly known. We performed a 2.5-D full kinetic simulation to study the energization of cold ions in a symmetric anti-parallel reconnection. We find that the cold ions can be considerably energized during reconnection. The average energy gain per cold ion is smaller than that per hot ion but larger than that per electron. The cold ions with the highest energy are firstly accelerated around the X-line, and further accelerated around the reconnection front in the outflow region. They perform the meandering motion at the X-line, while simultaneously accelerated by the reconnection out-of-plane electric field. The velocity distribution of cold ions around the X-line is characterized by the counter-streaming inflow component and the accelerated component in the +y quadrant. The cold ions are picked up at the reconnection front and trapped by the secondary island, resulting in a bump in the energy spectrum. The energy of the bump is close to the energy of the propagating speed of the reconnection front and the secondary island. Furthermore, a crescent-shaped velocity distribution is observed at the reconnection front, caused by the finite Larmor radius effect of the high-energy cold ions in the flux pileup region. Our result presents an important building block toward a fully understanding of the magnetosphere-ionosphere coupling.