Electron motion and distribution functions in the diffusion region of asymmetric reconnection

We study electron motion and electron distribution functions in the diffusion region in asymmetric reconnection with negligible guide field by means of two-dimensional particle-in-cell (PIC) simulations and theoretical analysis. Near the stagnation point of the electron inflows, which is located in the magnetospheric side in the diffusion region, electron distribution functions show two populations: A crescent-shaped component in the vy-vz plane (y is the out-of-plane direction, and z is the inflow direction), and the magnetospheric component that has been accelerated in the direction parallel to the magnetic field. Particle tracing in a PIC simulation shows that a population of electrons from the magnetosheath travels along crescent-shaped orbits in the velocity space during meandering motion across the neutral line. Those electrons are mainly accelerated by the in-plane electric field Ez, which is strong only in the magnetospheric side and plays a more significant role to energize electrons than the reconnection electric field. We theoretically predict the shape of the distribution functions near the stagnation point. The crescent-shaped distribution is formed because only a part of electrons from the magnetosheath can access a particular location in the magnetospheric side of the X-line. The penetration length of magnetosheath electrons into the magnetosphere is analytically derived. Theoretical predictions on the distribution shape and the penetration length agree well with simulation results. The crescent-shaped electron distributions are expected to be observed in dayside magnetopause reconnection by satellites, such as the NASA Magnetospheric Multi-scale (MMS) mission. We will discuss comparison between numerical and analytical results with data obtained by MMS as well as Cluster.