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When assessing the scattering of radiation belt electrons by fast magnetosonic (MS) waves, it is traditionally assumed that the waves follow the MS/whistler branch of the cold plasma dispersion relation (CPDR) in magnetohydrodynamics. However, MS waves are essentially ion Bernstein modes following a distinct kinetic dispersion relation. This study calculates the MS wave‐induced electron diffusion rates with the kinetic dispersion relation for the first time and compares the results with that obtained with the CPDR. It is found that the kinetic effects lead to a lower minimum resonant energy around 100 eV and a broader resonant pitch angle range. Kinetic effects also result in power spectral density attenuation when transforming wave frequency spectra into wavenumber spectra, so the diffusion rates are overall smaller than the ones obtained using the CPDR. Our results demonstrate that kinetic effects can significantly affect the role that MS waves play in the radiation belt dynamics. Plain Language Summary: Magnetosonic (MS) waves belong to the kinetic ion Bernstein modes essentially. But when the cold plasma is dominating, the waves also approximately follow the MS/whistler branch of the cold plasma dispersion relation (CPDR) in magnetohydrodynamics. Subsequently, studies of the electron scattering by MS waves have traditionally assumed the CPDR for simplicity. Motivated by recent studies which involved both satellite observations and kinetic theory revealing that the lower harmonic MS waves clearly follow the kinetic dispersion relation, we assess how the differences between the kinetic and cold plasma dispersion relations affect the MS wave‐induced electron scattering rates. Our results indicate that the kinetic dispersion relation produces relatively lower parallel phase speeds for MS waves, leading to a lower minimum resonant energy and subsequently a broader resonant pitch angle range for electrons (of a given energy). The kinetic effects also decrease the overall diffusion rates by attenuating the wave power spectral density in wavenumber space when mapped from a given frequency spectrum. Key Points: Linear kinetic dispersion relation indicates lower phase speeds and a broader range of group speeds for fast magnetosonic (MS) wavesThe lower phase speeds of MS waves result in a broader range of resonant pitch angles and lower minimum resonant energies of electronsKinetic effects reduce the wavenumber power spectral density and thus result in smaller electron diffusion rates [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]

Two‐dimensional hybrid particle‐in‐cell (PIC) simulations are carried out on a constant L‐shell (or drift shell) surface of the dipole magnetic field to investigate the generation process of near‐equatorial fast magnetosonic waves (a.k.a equatorial noise; MSWs hereafter) in the inner magnetosphere. The simulation domain on a constant L‐shell surface adopted here allows wave propagation and growth in the azimuthal direction (as well as along the field line) and is motivated by the observations that MSWs propagate preferentially in the azimuthal direction in the source region. Furthermore, the equatorial ring‐like proton distribution used to drive MSWs in the present study is (realistically) weakly anisotropic. Consequently, the ring‐like velocity distribution projected along the field line by Liouville's theorem extends to rather high latitude, and linear instability analysis using the local plasma conditions predicts substantial MSW growth up to ±27° latitude. In the simulations, however, the MSW intensity maximizes near the equator and decreases quasi‐exponentially with latitude. Further analysis reveals that the stronger equatorward refraction at higher latitude due to the larger gradient of the dipole magnetic field strength prevents off‐equatorial MSWs from growing continuously, whereas MSWs of equatorial origin experience little refraction and can fully grow. Furthermore, the simulated MSWs exhibit a rather complex wave field structure varying with latitude, and the scattering of energetic ring‐like protons in response to MSW excitation occurs faster than the bounce period of those protons so that they do not necessarily follow Liouville's theorem during MSW excitation. Key Points: The 2‐D hybrid PIC simulations are carried out on a constant L‐shell surface to simulate the excitation of MSWs for the first timeDespite the extended unstable region in latitude, MSWs do not grow well if they get latitudinally out of resonanceScattering of ring‐like protons during MSW excitation is local so that those protons do not necessarily follow Liouville's theorem [ABSTRACT FROM AUTHOR]

Electron Cyclotron Harmonic (ECH) waves driven by a loss cone distribution are studied in this work by self‐consistent particle‐in‐cell simulations. These waves have been suggested to play an important role in diffuse auroral precipitation in the outer magnetosphere. However, particle simulation of this instability is difficult because the saturation amplitude of the wave driven by a realistic size loss cone distribution is very small. In this work we use an extraordinarily large number of particles to reduce simulation noise so that the growth and saturation of ECH waves can be investigated. Our simulation results are consistent with linear theory in terms of growth rate, and with observation in terms of wave amplitude. We demonstrate that the heating of cold electrons is negligible and nonresonant, different from previous conclusions, and suggest that the saturation of the wave is caused by the filling of the loss cone of hot electrons. Plain Language Summary: Electron Cyclotron Harmonic (ECH) waves are electrostatic emissions in planetary magnetospheres with typical frequency between harmonics of electron cyclotron frequency. In terrestrial magnetosphere, these waves are believed to cause precipitation of energetic electrons into the atmosphere, forming diffuse aurora in the outer magnetosphere. However, detailed study of the excitation and saturation of the wave is difficult because the wave saturation amplitude driven by a realistic size loss cone distribution is typically below the simulation noise level. Correspondingly, a ring distribution was used in previous numerical studies of the wave. Here we show, by using an extraordinarily large number of simulation particles, the detailed excitation process of ECH waves and the associated electron dynamics. We demonstrate that, different from previous conclusions, the heating of cold electrons is both negligible and nonresonant. We also demonstrate the scattering of hot electrons into the loss cone and suggest that this process leads to the saturation of the wave. This study is useful to the further understanding of the importance of ECH waves in energetic electron dynamics. Key Points: Particle‐in‐cell simulation clearly shows the linear growth and saturation phases of ECH wavesSimulation demonstrates that heating of cold electrons is negligible and nonresonantSimulation suggests that ECH waves saturate by scattering hot electrons into the loss cone [ABSTRACT FROM AUTHOR]