The Origin of Low-Energy Anomalous Cosmic Rays at the Solar-Wind Termination Shock

We address the origin of the enhancement of ~40 keV to 5 MeV ions at the solar wind termination shock. Using self-consistent two-dimensional hybrid simulations (kinetic proton, fluid electron) of a shock moving through a plasma similar to that observed in the outer heliosphere, we conclude that the observed ion enhancements are consistent with accelerated "core" interstellar pickup ions (those that have not previously undergone any significant energization) by the termination shock via a combination of shock drift acceleration and particle scattering in meandering magnetic fields in the vicinity of the shock. In addition to the consequences for our understanding of anomalous cosmic rays, this work is also relevant to the more-general long-standing problem of accelerating low-energy particles by shocks that move nearly normal to a mean magnetic field.