Pickup ion ring-beam-driven instabilities and scattering in the outer heliosheath

Linear instability analysis and one-dimensional hybrid simulations are performed to investigate the plasma instabilities driven by the pickup ions in the outer heliosheath. The modes propagating parallel and anti-parallel to the background magnetic field are studied for pickup ion ring-beam distributions of different pickup angles (). The parallel thermal spread of the pickup ions is chosen to make the plasma parameters lie in the Alfven cyclotron stability gap to exclude the well-studied Alfven cyclotron instability. Our linear instability analysis reveals that the unstable modes all have left helicity but may propagate parallel or anti-parallel to the background magnetic field. At small , the parallel-propagating modes are unstable in two separate wavenumber (and frequency) ranges. The two unstable ranges merge into one when increases to 82. The simulation results first confirm the unstable modes predicted by the linear analysis and further reveal how the pickup ions are scattered by the excited waves. While the maximum growth rate of the low-frequency, left-helicity waves occurs at =82, the saturation level of the fluctuating magnetic field is the highest around =45. The subsequent scattering of the pickup ions is mostly in pitch angle and limited to the hemisphere of positive parallel velocity. For the pickup ion parameters examined, the pitch-angle spread of the pickup ions at the end of the simulations decreases with increasing pickup angle. Our study goes beyond previous studies of the pickup-ion-driven instabilities in the outer heliosheath which mainly focused on ring-like distributions of around pickup angle. The results have potential implications on the validity of the spatial retention scenario of the energetic neutral atom ribbon observed by the Interstellar Boundary EXplorer (IBEX).