Hybrid simulations of the ring-beam instabilities driven by the pickup ions in the outer heliosheath.

One-dimensional hybrid simulations are performed to investigate the plasma instabilities driven by the pickup ions in the outer heliosheath, which are believed to have a ring-beam velocity distribution in general. The modes propagating parallel and antiparallel to the background magnetic field are studied for pickup ion ring-beam distributions of different pickup angles, following the linear kinetic instability analysis presented in our previous work. 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 the pickup angle of 82° (consistent with the linear analysis results), the saturation level of the fluctuating magnetic field is the highest around 45° pickup angle. 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 90° pickup angle. With the caveats that the pickup ion parameters are in the Alfvén cyclotron stability gap and that the one-dimensional simulations only include parallel and antiparallel-propagating modes, our results show that the scattering of pickup ions at small pickup angles is limited to one hemisphere so they can drift away from the ribbon direction as required by the spatial retention scenario of the energetic neutral atom ribbon observed by the Interstellar Boundary EXplorer. [ABSTRACT FROM AUTHOR]