Recently, a team led by Prof. Kaijun Liu (Earth and Space Sciences, SUSTech) made significant progress in the excitation of oxygen ion cyclotron harmonic waves in the inner magnetosphere. The results have been published in Geophysical Research Letters, one of the top journals in space physics.

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. Recently, hybrid simulations were carried out by Prof. Kaijun Liu’s team to investigate the excitation of these waves. Their 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, their three-dimensional simulation results convincingly demonstrate that, while the excited waves have quasi-perpendicular propagation, the superposition of multiple waves with different azimuthal angles has caused the SVD method to yield small wave normal angles, incorrectly. Finally, 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.

Oxygen ion cyclotron harmonic wave is one type of plasma fluctuations observed in the Earth’s inner magnetosphere. The waves have discrete spectral peaks at or near multiple harmonics of the oxygen ion cyclotron frequency and, therefore, earned the name “oxygen ion cyclotron harmonic waves.” Linear plasma theory suggests that these waves can be excited by energetic oxygen ions of ring-like velocity distribution. The resultant waves have properties consistent with observations except that the waves propagate nearly perpendicular to the background magnetic field according to linear theory, in contrast to the nearly parallel propagation given by the standard SVD method when analyzing the observed wave magnetic field data.

Figure 1. The power spectrum of magnetic field fluctuations given by a one-dimensional hybrid simulation.

The research team led by Prof. Kaijun Liu executed self-consistent hybrid simulations to investigate the excitation of oxygen ion cyclotron harmonic waves. As shown in Figure 1, their one-dimensional simulation nicely confirms the prediction of linear instability analysis that waves at multiple harmonics of the oxygen ion cyclotron frequency can be excited by energetic oxygen ions of ring-like velocity distribution. More importantly, their three-dimensional simulation demonstrates that, while the excited waves have quasi-perpendicular propagation as shown in Figure 2，the superposition of the enhanced waves with different azimuthal angles breaks the plane-wave assumption of the SVD method and has caused the method to yield, incorrectly, small wave normal angles (as shown in Figure 3b).

In addition, the results show that the enhanced waves interact effectively with oxygen ions through cyclotron resonance and lead to significant transverse heating of the cool background oxygen ions. The waves may also contribute to the radiation belt electron dynamics by scattering them through bounce resonance and transit time scattering, similar to fast magnetosonic waves.

Figure 2. Isosurface plot of the wave magnetic field power spectrum in wave number space shows that the excited waves propagate nearly perpendicular to the background magnetic field.

Figure 3. (a) Spectrogram of the wave magnetic field and (b) the corresponding wave normal angles calculated using the SVD method.

The work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences, NSFC Grant, and Shenzhen Science and Technology Program.

Link to the paper: https://doi.org/10.1029/2020GL090575