Generation Mechanism of Oxygen Ion Cyclotron Harmonic Waves in the Inner Magnetosphere: Linear Instability Analysis Based on Observations

Linear kinetic instability analysis based on in situ observations for one typical oxygen ion cyclotron harmonic (OCH) wave event is performed to investigate the wave excitation mechanism. The observed partial‐shell velocity distribution of energetic O+s is fitted by the superposition of multiple ring‐beam distributions. The calculated linear growth rate shows that the observed OCH waves can be excited by energetic O+s with the partial‐shell velocity distribution, and the maximum growth rate occurs at the third harmonic, coinciding with the frequency associated with the maximum peak of the wave electric field power spectral density measured. In addition, it is found that cool heavy ions have a damping effect on the OCH waves near the corresponding heavy ion cyclotron frequency. Plain Language Summary Oxygen ion cyclotron harmonic (OCH) waves observed in the terrestrial magnetosphere have multiple spectral peaks at harmonics of the local oxygen ion cyclotron frequency. This paper presents the linear kinetic instability analysis based on in situ observations of the plasma environment and ion distributions for one typical OCH wave event. The results reveal that partial‐shell distributed energetic O+s can provide free energy to excite the observed OCH waves. The growth rate versus frequency calculated using linear kinetic theory shows good agreement with the power spectral density of the observed wave electric field. Interestingly, damping of the OCH waves of the harmonics close to the oxygen (helium) ions cyclotron frequency is positively correlated with the concentration of the cool oxygen (helium) ions. Key Points Linear analysis based on observations demonstrates that partial‐shell distributed energetic O+s excite oxygen ion cyclotron harmonic waves Maximum growth rate occurs at the third harmonic, coinciding with the frequency at the largest peak of the observed power spectral density The presence of cool heavy ions damps the wave harmonics near the heavy ion cyclotron frequency