Hybrid Simulations of Interstellar Pickup Protons Accelerated at the Solar-wind Termination Shock at Multiple Locations

We estimate the intensity of interstellar pickup protons accelerated to ∼50 keV at various locations along the solar-wind termination shock, using two-dimensional hybrid simulations. Parameters for the solar wind, interstellar pickup ions (PUIs), and magnetic field just upstream of the termination shock at one flank of the heliosphere, and at the location in the downwind (or tail-ward) direction are based on a solar-wind/pickup-ion/turbulence model. The parameters upstream of the shock where Voyager 2 crossed are based on observations. The simulation is limited in size, and therefore cannot accurately model the distribution to energies much beyond ∼50 keV. This is sufficient to study the origin of the high-energy tail of the distribution, which is the low-energy portion of the anomalous cosmic-ray spectrum. We also extrapolate our results to other locations along the termination shock, such as the other flank, and the poles of the heliosphere. We find that the intensity of ∼10–50 keV accelerated pickup protons is remarkably similar at all three locations we simulated, suggesting that particles in this energy range are relatively uniformly distributed along the termination shock, and are likely quite uniform throughout the entire heliosheath. In addition, we find significant differences in the distribution in the 0.5–1 keV energy range for energetic neutral atoms coming from the tail region of the heliosphere compared to that at the nose or flank look directions. This is because the peak in the PUI distribution is at a higher energy there.