Evolution of a Pickup Ion Ring-beam Population in Large-scale Interstellar Turbulence and Small-scale Self-generated Waves: Implications for the IBEX Ribbon

The secondary energetic neutral atom (ENA) mechanism proposed to explain the ribbon-like enhancement of ENA emission observed by Interplanetary Boundary Explorer (IBEX) involves a pickup ion (PUI) ring-beam population beyond the heliopause. The stability of this highly anisotropic PUI distribution is crucial for determining the physical process responsible for the ribbon formation. It has been proposed that PUIs with pitch angles near 90° can become trapped via the magnetic mirroring process in the turbulent magnetic field of the local interstellar medium. However, the turbulent mirroring scenario typically neglects self-generated waves excited by the kinetic instabilities of the ring-beam PUIs, which can efficiently scatter PUIs in pitch angle. Using a magnetohydrodynamics-particle-in-cell approach implemented in ATHENA++, we simulate the evolution of the ring-beam PUIs, incorporating both large-scale interstellar magnetic fluctuations and microscale self-excited instabilities. We examine the competition between turbulent mirroring and pitch-angle scattering by the self-generated waves. Our results show that the PUIs can be rapidly scattered toward a nearly isotropic distribution on the order of one day, much shorter than their characteristic lifetime of a couple of years. This suggests that theoretical difficulties remain for maintaining an anisotropic PUI distribution in physical models of the IBEX ribbon.