Excitation of Oxygen Ion Cyclotron Harmonic Waves in the Inner Magnetosphere: Hybrid Simulations

• Published in: Geophysical research letters 2020-10, Vol.47 (20), p.n/a
• Main Authors: Liu, Kaijun, Min, Kyungguk, Feng, Bolu, Wang, Yan
• Format: Article
• Language: English
• Subjects: Analysis; Cyclotron frequency; Cyclotron resonance; Decomposition; Distribution; electron scattering; Electrons; Excitation; hybrid simulation; ion Bernstein wave; Ion cyclotron waves; Ions; Magnetism; Magnetospheres; Oxygen; oxygen ion cyclotron harmonic wave; Oxygen ions; Propagation; Properties; Radiation; Radiation belt electrons; Radiation belts; Resonance; Resonance scattering; Scattering; Simulation; Singular value decomposition; singular value decomposition method; Transit time; Velocity; Velocity distribution; Wave analysis; Wave propagation
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2020GL090575

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 distribution Excited waves have nearly perpendicular propagation but their superposition causes wave polarization analysis to yield wrong propagation The waves heat the cool O+ transversely and can scatter radiation belt electrons t