Excitation of Oxygen Ion Cyclotron Harmonic Waves in the Inner Magnetosphere

Oxygen ion cyclotron harmonic waves, with discrete spectral peaks at multiple harmonics of the oxygen ion cyclotron frequency, have been observed in the inner magnetosphere. Their excitation mechanism has remained unclear, because the singular value decomposition (SVD) method commonly used in satellite wave data analysis suggests that the waves have quasi-parallel propagation, whereas plasma theory reveals unstable modes at nearly perpendicular propagation. Hybrid simulations are carried out to investigate the excitation of these waves. The simulation results show that waves at multiple harmonics of the oxygen ion cyclotron frequency can be excited by energetic oxygen ions of a ring-like velocity distribution. More importantly, analyzing the simulated waves in a three-dimensional simulation using the common SVD method demonstrates that, while the excited waves have quasi-perpendicular propagation, the superposition of multiple waves with different azimuthal angles causes the SVD method to yield incorrectly small wave normal angles. In addition, the scattering of oxygen ions by the excited waves is examined in the simulations. The waves can cause significant transverse heating of the relatively cool background oxygen ions, through cyclotron resonance. The waves may also scatter energetic radiation belt electrons through bounce resonance and transit time scattering, like fast magnetosonic waves.