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The scattering of charged particles by oxygen ion cyclotron harmonic (OCH) waves in the inner magnetosphere is investigated by evaluating the relevant quasi‐linear diffusion coefficients. Recent studies demonstrated that OCH waves are oxygen ion Bernstein modes and their complex kinetic dispersion relation has made it challenging to assess their role in scattering charged particles. The present study calculates the quasi‐linear diffusion coefficients for the scattering of electrons and ions by OCH waves using their kinetic dispersion relation. The results show that OCH waves can effectively scatter electrons between ∼100 eV and 100s keV via Landau resonance. They are also capable of heating cold helium and oxygen ions through cyclotron resonances. Specially, it is found that the 4th harmonic of OCH waves can lead to effective heating of helium ions, while oxygen ions would interact more efficiently with lower harmonics of OCH waves. Plain Language Summary: Oxygen ion cyclotron harmonic (OCH) waves observed in the inner magnetosphere often have multiple spectral peaks at harmonics of the local oxygen ion cyclotron frequency. They have been shown to be excited by hot oxygen ion loss‐cone or ring/ring‐like distributions and follow a complicated kinetic dispersion relation for oxygen ion Bernstein waves. Since OCH waves cannot be described by the relatively simple cold plasma dispersion relation, it has been difficult to calculate their diffusion coefficients in scattering charged particles in quasi‐linear theory. The present study numerically solves the kinetic dispersion relation for OCH waves and then uses the results to calculate the corresponding quasi‐linear diffusion coefficients for electrons and ions. The diffusion coefficients obtained show that OCH waves can effectively interact with ∼100 eV to 100s keV electrons and are capable of heating cold helium and oxygen ions. Thus, OCH waves have their own unique contribution to the particle dynamics in the inner magnetosphere. Key Points: Quasi‐linear diffusion coefficients are evaluated for particle scattering by oxygen ion cyclotron harmonic (OCH) waves for the first timeOCH waves can scatter electrons in a wide energy range (∼100 eV–100s keV) via Landau resonanceOCH waves are capable of heating cold helium and oxygen ions through cyclotron resonance [ABSTRACT FROM AUTHOR]

Linear instability analysis as well as a corresponding two‐dimensional hybrid simulation are performed to examine the excitation of the proton cyclotron waves observed upstream of Mars. The waves are believed to be excited by the pickup ions produced from the ionization of the Martian hydrogen exosphere. And previous statistical analysis of wave observations suggested that the waves are mostly related to pickup ion beam velocity distributions. While earlier linear instability analysis of pickup ion beam distributions has mainly been focused on the parallel unstable modes, our analysis reveals that the maximum growth rate occurs at very oblique propagation. The corresponding hybrid simulation confirms the linear analysis results and further demonstrates that the pickup ions are scattered toward an isotropic shell velocity distribution by the waves excited. Interestingly, the waves at oblique propagation gradually damp out and the system is eventually dominated by waves of quasi‐parallel propagation. Plain Language Summary: Waves near the local proton cyclotron frequency have commonly been observed upstream of Mars. Without the shielding of a global intrinsic magnetic field, the Martian exosphere interacts directly with the solar wind. Pickup ions are generated when neutrals in the exosphere are ionized and "picked up" by the solar wind. Observations suggested that the proton cyclotron waves upstream of Mars are excited by the newborn pickup ions, especially when they have a large drift along the background magnetic field with respect to the solar wind ions (i.e., a beam velocity distribution). The present study carries out linear instability analysis for pickup ions of a beam distribution. The results reveal the most unstable wave at large propagation angle, in contrast to previous analysis mainly focused on parallel unstable modes. Our subsequent hybrid simulation confirms the linear theory prediction and demonstrates the scattering of the pickup ions by the enhanced waves. More importantly, although the oblique waves grow first, they also fade away faster so that the simulation is dominated by waves at quasi‐parallel propagation in the late stage. Key Points: Pickup ion beam distribution can generate the left‐hand polarized proton cyclotron waves observed upstream of Mars after Doppler shiftThe maximum growth rate occurs at oblique propagation instead of parallel propagationEnhanced waves pitch angle scatter pickup ions toward an isotropic shell velocity distribution [ABSTRACT FROM AUTHOR]

Scattering of pickup ion ring-beam distributions in the outer heliosheath is a fundamental element in the spatial retention scenario of the energetic neutral atom (ENA) ribbon observed by the Interstellar Boundary EXplorer (IBEX). According to our earlier linear instability analysis, pickup ion ring-beam distributions trigger magnetic field-aligned, right-hand polarized unstable waves in two separate frequency ranges which are near and far above the proton cyclotron frequency, respectively. We have performed hybrid simulations to study the unstable waves near the proton cyclotron frequency. However, the high-frequency waves well above the proton cyclotron frequency are beyond the reach of hybrid simulations. In this paper, particle-in-cell simulations are carried out to study the parallel- and antiparallel-propagating high-frequency waves excited by the outer heliosheath pickup ions at different pickup angles as well as the scattering of the pickup ions by the waves excited. In the early stages of the simulations, the results confirm the excitation of the parallel-propagating, right-hand polarized high-frequency waves as predicted by the earlier linear analysis. Later in the simulations, enhanced antiparallel-propagating modes also emerge. Furthermore, the evolution of the pickup ion ring-beam distributions of the selected pickup angles reveals that the high-frequency waves do not significantly contribute to the pickup ion scattering. These results are favourable regarding the plausibility of the spatial retention scenario of the IBEX ENA ribbon. [ABSTRACT FROM AUTHOR]

One-dimensional hybrid simulations are performed to investigate the plasma instabilities driven by the pickup ions in the outer heliosheath, which are believed to have a ring-beam velocity distribution in general. The modes propagating parallel and antiparallel to the background magnetic field are studied for pickup ion ring-beam distributions of different pickup angles, following the linear kinetic instability analysis presented in our previous work. The simulation results first confirm the unstable modes predicted by the linear analysis and further reveal how the pickup ions are scattered by the excited waves. While the maximum growth rate of the low-frequency, left-helicity waves occurs at the pickup angle of 82° (consistent with the linear analysis results), the saturation level of the fluctuating magnetic field is the highest around 45° pickup angle. For the pickup ion parameters examined, the pitch-angle spread of the pickup ions at the end of the simulations decreases with increasing pickup angle. Our study goes beyond previous studies of the pickup-ion-driven instabilities in the outer heliosheath which mainly focused on ring-like distributions of around 90° pickup angle. With the caveats that the pickup ion parameters are in the Alfvén cyclotron stability gap and that the one-dimensional simulations only include parallel and antiparallel-propagating modes, our results show that the scattering of pickup ions at small pickup angles is limited to one hemisphere so they can drift away from the ribbon direction as required by the spatial retention scenario of the energetic neutral atom ribbon observed by the Interstellar Boundary EXplorer. [ABSTRACT FROM AUTHOR]

Hybrid simulations are carried out to investigate the excitation of oxygen ion cyclotron harmonic waves observed in the inner magnetosphere. The simulations show that these waves can be excited by energetic oxygen ions of a ring‐like velocity distribution. The excited waves have properties consistent with observations, but their nearly perpendicular propagation disagrees with the quasi‐parallel propagation from the singular value decomposition (SVD) analysis reported earlier. By performing the same SVD analysis on the simulated waves in a three‐dimensional simulation, it is demonstrated that the superposition of multiple waves with different azimuthal angles can cause the commonly used SVD analysis of wave propagation to yield incorrectly small wave normal angles. In addition, the results show that the enhanced waves scatter oxygen ions mainly along v⊥ through cyclotron resonance. The waves may also scatter energetic radiation belt electrons through bounce resonance and transit time scattering, similar to fast magnetosonic waves. Plain Language Summary: Waves at multiple harmonics of the oxygen ion cyclotron frequency (known as oxygen ion cyclotron harmonic waves) have been observed in the inner magnetosphere. Their source is still under debate because observations suggest they have quasi‐parallel propagation, whereas linear theory reveals unstable modes (of otherwise similar properties) at nearly perpendicular propagation. The present one‐dimensional hybrid simulation first demonstrates that the waves can be excited by energetic oxygen ions of a ring‐like velocity distribution. More importantly, our three‐dimensional simulation shows that the superposition of multiple waves with different azimuthal angles can make the commonly used singular value decomposition method to give false estimate of wave normal angles. Finally, the scattering of ions by the waves excited are examined. The waves can cause efficient transverse heating of the cool background oxygen ions. They might also be capable of scattering the energetic radiation belt electrons through bounce resonance and transit time scattering. Key Points: Hybrid simulations demonstrate that oxygen ion cyclotron harmonic waves can be excited by energetic oxygen ions of a ring‐like distributionExcited waves have nearly perpendicular propagation but their superposition causes wave polarization analysis to yield wrong propagationThe waves heat the cool O+ transversely and can scatter radiation belt electrons through bounce resonance and transit time scattering [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]