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

Abstract 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.