Phase-matched ion pre-acceleration by upstream magnetic dipole vortices in relativistic collisionless shocks

Collisionless shocks are among the most efficient particle accelerators in the Universe, yet the physical mechanism responsible for energetic upstream seed ions remains poorly understood. Using two-dimensional particle-in-cell simulations of unmagnetized relativistic electron-ion shocks, we investigate ion pre-acceleration by self-generated magnetic dipole vortices (MDVs) in the upstream region of the shock. We identify two distinct ion populations: one remains phase-matched with the moving MDV, undergoes considerable energization, and returns toward the shock, whereas the other escapes upstream after gaining only modest energy. We show that the magnetic component of the Lorentz force reduces the ion’s longitudinal velocity relative to the moving MDV, extending the interaction time and allowing the transverse electric field to perform sustained positive work. Decomposition of the electric field shows that both the motional electric field associated with the moving MDV and plasma-generated fields contribute to the observed ion energization. Repeated interactions with successive MDVs can increase ion energy before the particles return to the shock transition region. These results identify MDV-mediated upstream ion pre-acceleration as a candidate mechanism for producing energetic seed ions that may facilitate subsequent shock acceleration.