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

The consequences of injection of pickup protons perpendicular to a background magnetic field in a homogeneous, collisionless plasma are studied using one-dimensional hybrid simulations. Freshly ionized protons are continuously injected into the simulations at constant rates and relative speeds approaching conditions in the outer heliosheath. The pickup protons initially form a ring-velocity distribution unstable to the electromagnetic proton cyclotron instability which leads to enhanced magnetic fluctuations near and below the proton cyclotron frequency. The magnetic fluctuations first have a quiescient phase, followed by a phase of exponential growth. There is little proton scattering during the quiescient phase, but rapid pitch-angle scattering of the pickup protons toward an isotropic velocity shell distribution during the exponential growth phase. The pickup proton density at onset of rapid scattering increases with the pickup proton injection rate; scaling relations for the onset density and the subsequent scattering rate are derived from the simulations. These results suggest that significant scattering of pickup protons in the outer heliosheath occurs in a relatively limited spatial regime close to the heliopause.