The Energization and Escape of Cold Ions in Dayside Magnetopause Magnetic Reconnection

Abstract

At the Earth's dayside magnetopause, a cold ion population of ionospheric or plasmasphere origin is commonly observed at the magnetospheric side. In this study we use a 2.5D Particle-in-Cell simulation to investigate the energization of cold ions in the separatrix near X-line and the escape process. And we identify observation events made by the Magnetospheric Multiscale mission, which provide evidence of the acceleration mechanism of cold ions in separatrix. We track the trajectories of cold ions and conduct an analysis, discovering that the cold ions exhibit a positive drift velocity in the vertical direction of the current sheet and $E+v_{ci}\times B\ne 0$ in the separatrix, so the cold ions undergo demagnetized motion. The analysis results show that the Hall electric field accelerates the cold ions, and it is followed by gyrations around the magnetic field, which results in the velocity distribution function of cold ions near the separatrix exhibits a ring-like distribution. Both simulation results and observations indicated that cold ions in the asymmetric magnetic reconnection separatrix region near the X-line undergo significant acceleration owing to the effects of $E\times B$, where Hall electric field and the magnetic field parallel to the current sheet play the primary roles during this process. Although magnetic reconnection opens a channel for cold ion escape, the escape is limited, and we first predict the quantification of the escape rate of the cold ions, approximately $0.2\%$. The escape of cold ions from the magnetosphere to the magnetosheath is challenging and rare.