Kinetics of Cold Ions in Asymmetric Reconnection: Particle-In-Cell Simulation

Cold ions from Earth's ionosphere and plasmasphere have frequently been observed at the magnetopause, where they impact reconnection and get energized in the process. The behavior of cold ions in different regions of magnetopause reconnection is not well understood. This study investigates their kinetics in asymmetric reconnection through a 2.5-D fully kinetic simulation. The simulation starts with cold ions being present only in the magnetosphere. We find that the velocity distribution functions (VDFs) of cold ions in different reconnection regions are composed of two types of particles, distinguished by their ability to penetrate the magnetosheath. One type produces ring-shaped distributions in the magnetosheath due to meandering motion, while the other generates ring-shaped distributions in the magnetospheric separatrix as a result of gyromotion. The reconnection electric field Ey has a negative effect on one type and a positive effect on the other. Moreover, the Hall electric field Ez can stepwise accelerate the meandering particles. The cold ion temperature increases significantly in the magnetosheath as compared to its original temperature, the enhancement is a kinetic effect as a result of the ring-shaped velocity distribution. On the other hand, in the remaining regions, cold ions experience bulk heating, resulting in noticeable temperature elevation. These findings further emphasize the necessity to examine the VDF for an all-round comprehension of the particle heating mechanism and contribute to a better understanding of cold ion dynamics at the Earth's magnetopause.