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Mirror Instability Driven by Pickup Ions in the Outer Heliosheath

The stability of the pickup ions in the outer heliosheath is a vital factor in the generation of the energetic neutral atom (ENA) ribbon observed by the Interstellar Boundary EXplorer according to the secondary ENA mechanism. Most previous studies of the pickup ion stability assumed simple, idealized velocity distributions of the pickup ions and focused on the parallel-propagating modes only. This paper takes a more realistic multicomponent pickup ion velocity distribution given by the global modeling of neutral atoms in the heliosphere and investigates the role of the oblique mirror waves, in addition to the parallel modes. Both linear kinetic instability analysis and hybrid simulations are performed. In contrast to a recent investigation using the same distribution that demonstrated the growth of parallel waves but reported an insignificant contribution of oblique modes, our study reveals substantial growth of the oblique mirror instability. The oblique mirror modes and the parallel/quasi-parallel ion cyclotron waves grow simultaneously with different growth rates. The pickup ion scattering by two types of excited waves together is more pronounced than by either type of wave alone. More importantly, our two-dimensional hybrid simulation results demonstrate that the development of the mirror instability not only produces its own pickup ion scattering, but also leads to the growth of extra ion cyclotron waves (in a quasi-linear manner), which further enhances the pickup ion scattering. The results suggest an important role of the mirror modes that should not be ignored in the stability study of the outer heliosheath pickup ions.