Scattering of Continuously Injected Pickup Ions of Oblique Pickup Angles in the Outer Heliosheath

Pitch angle scattering of pickup ions (PUIs) in the outer heliosheath is studied using two-dimensional hybrid simulations, with the PUIs continuously injected at specified injection rates and pickup angles. The simulations reveal the growth of quasiparallel-propagating waves, quasi-antiparallel-propagating waves, and the oblique mirror mode. The excited waves scatter the PUIs and can reverse their parallel velocities with respect to the background magnetic field. The scattering time, defined as the time when the squared standard deviation of the pitch angles of the PUIs approaches thresholds of 0.2 or 0.4, increases as the PUI injection rate decreases, approximately following a power law. The scattering time estimated for a realistic injection rate in the outer heliosheath is about 0.5 yr at a 45° pickup angle. This scattering time is shorter than the average charge exchange time of about 2 yr for the PUIs to be converted into energetic neutral atoms (ENAs). This result conflicts with the spatial retention scenario of the secondary ENA mechanism, which assumes weak (or incomplete) scattering of PUIs in the off-ribbon directions to explain the ENA ribbon discovered by the Interstellar Boundary Explorer.