tokens: the combined energy range of 10 eV–6 keV. The lower energy corresponds to proton velocities of about 44 km s −1 . This means that pristine LISM neutral H atoms cannot be seen by IBEX unless it moves in the direction opposite to the LISM (Heerikhuisen Pogorelov 2011 ). The highest energy corresponds to velocities of about 1000 km s −1 . Bulk velocities of this magnitude are not observed during SW–LISM interaction, except perhaps in coronal mass ejections. However, thermal velocities of this order are possible beyond the TS. An interesting feature discovered by IBEX is the narrow region of enhanced ENA flux, which is now referred to as the IBEX ribbon. Before any explanation of this phenomenon was proposed, it was noticed that the directions toward the ribbon correlated with the line-of-sight directions perpendicular to the ISMF lines draping around the HP in the simulation of Pogorelov et al. ( 2009 ). One of the possible explanations of the ribbon flux was mentioned by McComas et al. ( 2009b ) and Schwadron et al. ( 2009 ), and described in more detail by Heerikhuisen et al. ( 2010 ). It is based on the assumption that PUIs born in the LISM region in front of the HP due to charge exchange of the neutral SW H atoms with interstellar ions, if injected with a pitch angle close to 90° to the draped ISMF direction, preserve their anisotropy long enough to produce ENAs propagating toward IBEX in the plane perpendicular to . This scenario is supported by numerical simulations, when anisotropy is assumed. We also note that the relative brightness of the ribbon is highest at energy ∼1.1 keV, which corresponds to the speed of slow SW of roughly 460 km s −1 . In this paper, we test the crucial assumption of this IBEX ribbon scenario, namely that the anisotropy of a realistic PUI distribution can be preserved long enough to experience another charge exchange. We use fully kinetic particle-in-cell (PIC) simulations to investigate the time evolution of the PUI distribution function in the outer heliosheath (outer) just outside of the HP. The initial distribution function is derived from the neutral H simulations performed by Heerikhuisen et al. ( 2016 ) and takes into account PUIs of the neutral SW and IHS. 2. PUIs in the Outer Heliosphere The ribbon mechanism developed by Heerikhuisen et al. ( 2010 ) requires that the PUI distribution function in the OHS preserves a certain degree of anisotropy for a sufficiently long time for a subsequent charge exchange to occur (∼2 yr for 1.1 keV H ions). This is not quite obvious because ring-beam distributions, typical for ions externally inserted into a background plasma, tend to scatter onto a shell distribution (Williams Zank 1994 ). Heerikhuisen et al. ( 2010 ) assumes that the ring-beam distribution is scattered onto a partial shell distribution depending on the injected pitch angle. In contrast, Chalov et al. ( 2010 ) neglects scattering, but appeals to magnetic mirroring effects that ensure spatial localization
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