Authors: Justin Riggs (University of Iowa), Gregory G. Howes (University of Iowa)

Collisionless shocks play a central role in energy conversion in space plasmas throughout the heliosphere. A key mechanism for particle acceleration at shock fronts involves the reflection of a fraction of the incoming ion population back upstream. Under the collisionless conditions typical of space plasmas, this reflected ion population must be created by the two key components of the shock structure: the cross-shock electric field and the jump in the transverse magnetic field. We investigate the relative contributions of these two fields to ion reflection, in the shock rest, normal incidence frame, by calculating the impulse across phase space using analysis of single ion trajectories and Liouville mapping. We employ both hybrid numerical simulation shock fields and an empirical quasiperpendicular shock model spanning Alfvén Mach numbers 4 to 16, and shock normal angles 45° to 90°. We find that the contribution of the magnetic forces to the reflection of ions increases with the Alfvén Mach number and is less strongly impacted by the changes in shock normal angle. Our results using the numerical simulation electromagnetic fields indicate that the magnetic field dominates ion reflection when the Alfvén Mach number is greater than approximately 6.