Ion Reflection at Collisionless Shocks: The Competing Roles of Electric and Magnetic Fields

Collisionless shocks are locations of complex energy conversion in space plasmas. A key part of particle acceleration at shock fronts is the reflection of a portion of the incoming ion population back upstream. This reflected ion population is primarily created by two electromagnetic field components within the shock transition region: the cross-shock electric field and the transverse magnetic field. In the normal-incidence shock-rest frame of reference, we investigate the relative contributions of the electric and magnetic forces to the total ion reflection by calculating their impulse on the ions using single-particle motion tracing. We employ a suite of 16 hybrid numerical simulation to determine the electromagnetic fields through the shock as well as an empirical shock model created from those simulations. The simulations span Alfvén Mach numbers from 4 to 16 and shock-normal angles from 45° to 90°. We find that increasing Alfvén Mach number enhances the fraction of ions reflected via the magnetic force, while variations in the shock-normal angle do not have a strong effect. Our results indicate that the magnetic field dominates ion reflection for Alfvén Mach number greater than 7 in the numerical simulation fields and greater than 4.5 in the empirical model fields. These results point toward the magnetic field being largely responsible for the creation of the reflected ion population upstream of Earth's bow shock when these conditions arise.