Mirror instability driven by pickup ions in the outer heliosheath

Abstract:

The ribbon of energetic neutral atoms (ENAs) observed by the Interstellar Boundary EXplorer (IBEX) relies on the stability of the pickup ions in the outer heliosheath according to the secondary ENA mechanism. While most previous investigations of the pickup ion stability considered idealized velocity distributions of the pickup ions and only emphasized the parallel-propagating modes, this paper takes a more realistic multi-component pickup ion velocity distribution given by the global modeling of neutral atoms in the heliosphere. More importantly, the role played by the oblique mirror waves is also addressed using both linear kinetic instability analysis and hybrid simulations. Our results reveal substantial growth of the oblique mirror instability in addition to the usual parallel propagating modes, whereas a recent investigation using the same distribution reported an insignificant contribution of oblique modes and only highlighted the growth of the later modes. Our two-dimensional hybrid simulation shows that the scattering of pickup ions by the simultaneous excitation of the oblique mirror and parallel/quasi-parallel ion cyclotron modes is more pronounced than by either of these two modes alone. Interestingly, the scattering of pickup ions by the mirror instability subsequently leads to the excitation of extra ion cyclotron waves in a quasi-linear manner, which then further increase the scattering of pickup ions. Overall, this study shows that it is crucial to take into account the mirror instability in studying the dynamics of the outer heliosheath pickup ions.