Pickup proton instabilities and scattering in the distant solar wind and the outer heliosheath: Hybrid simulations

The growth of magnetic field fluctuations driven by the injection of pickup ions perpendicular to a background magnetic field in a homogeneous, collisionless plasma is studied using one-dimensional hybrid simulations. Freshly ionized protons are continuously injected into the simulations at constant rates and relative speeds consistent with conditions in the distant solar wind and the outer heliosheath. The pickup protons initially form a ring-velocity distribution unstable to the electromagnetic proton cyclotron instability and lead to enhanced magnetic fluctuations. After an exponential growth phase of the instability, the fluctuating magnetic fields exhibit linear temporal growth followed by a more-slowly-growing quasi-steady phase. The excited fluctuations pitch-angle scatter the pickup protons toward an isotropic shell velocity distribution with the most significant scattering occurring in the exponential growth phase. The scattering rate of the freshly-injected pickup protons during the linear temporal growth phase remains relatively constant and it increases with the pickup proton injection rate. More importantly, significant pitch-angle scattering only occurs after the accumulated pickup proton density exceeds a critical value, the scattering-onset density. The scattering-onset density also increases with the pickup proton injection rate and the scattering onset typically occurs during the exponential growth phase of the magnetic fluctuations. Scaling relations for the scattering rate and the scattering-onset density versus the pickup proton injection rate are derived from the simulation results. These relations suggest that significant scattering of pickup protons in the outer heliosheath occurs in a relatively limited spatial range close to the heliopause, related to the issue of whether the "secondary ENA" mechanism is a possible explanation for the ENA ribbon observed by IBEX. Implications of the results on pickup proton dynamics in the distant solar wind are also discussed.