Pickup Ions beyond the Heliopause

Fluxes of energetic neutral atoms (ENAs) with energies greater than 1 KeV measured by the Interstellar Boundary Explorer (IBEX) are predominantly created by charge exchange of interstellar neutral atoms with hot, nonthermal (pickup) ions. Since the properties of pickup ions (PUIs) depend on the place of their birth, they bear an imprint of the interaction between the solar wind (SW) and the local interstellar medium (LISM). Of special importance is a narrow "ribbon" of an enhanced ENA flux discovered by IBEX. While the origin of this ribbon is still under debate, various models attribute it to charge exchange between the LISM neutral atoms and PUIs just beyond the heliopause. In this work, we investigate the stability of PUI distributions inferred from global modeling of neutral atoms in the heliosphere using fully kinetic particle-in-cell modeling. The initial distribution consists of PUIs created from high-speed, low-temperature neutral atoms born in the supersonic SW ahead of the heliospheric termination shock (TS) and PUIs created from low-speed, high-temperature H atoms born between the TS and the heliopause. The simulations indicate that such distributions are unstable against quasi-parallel modes with fast growth timescales (of the order of one hour or less). The development of these instabilities may strongly modify the initial PUI distribution, leading, for example, to formation of a plateau in the direction parallel to the local magnetic field. These modes, however, saturate at relatively small amplitudes and do not lead to complete isotropization of the distribution on the timescales considered.