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

Cold ions from Earth's ionosphere and plasmasphere are frequently observed at the Earth's magnetopause, impacting reconnection in ways such as altering the reconnection rate and energy budget. Despite extensive research on reconnection, the fluid properties and kinetics of cold ions in magnetopause reconnection remain unclear. Our recent 2-D particle-in-cell simulation provides new insight into cold ion dynamics in asymmetric reconnection. Our simulation shows that cold ions, initially located only in the magnetosphere, absorb 10% to 25% of the total released magnetic energy, primarily converting it into thermal energy through stochastic heating, that is, the viscous heating associated with the non-gyrotropic pressure tensor. Cold ions are step-by-step accelerated by the Hall electric field during meandering motion across the magnetopause current sheet. The velocity distribution functions of cold ions in different regions are made up of two types of particles, differentiated by their ability to penetrate into the magnetosheath. The reconnection electric field has different effects on these two types of cold ions. As reconnection continues, the velocity distribution functions of the cold ions diffuse, leading to bulking heating. These findings significantly enhance our understanding of cold ion dynamics at the Earth's magnetopause.