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

Justin Riggs

Collisionless shocks are locations of complex energy conversion in space plasmas due to the lack of contact between particles. A key part of particles being 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 the two electromagnetic fields within the shock transition region: the cross-shock electric field and the magnetic field gradient. We investigate the relative contributions of these two fields to the total reflected ion population, in the normal incidence shock rest frame, by mapping impulse in phase space using a single-particle motion analytical code. We employ a suite of 16 hybrid numerical simulation shock fields, and an empirical shock model created from those simulations, spanning Alfvén Mach numbers 4 < M_A <16 and shock normal angles 45° <θ_BN <90°. We find that increasing MA enhances the fraction of ions reflected via the magnetic braking force, while variations in θ_BN do not have as significant an effect. Our results indicate that the magnetic field dominates ion reflection for M_A >7 in the numerical simulation shock field profiles and for M_A >4.5 in our empirical model shock 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.