“…In the past few years, harmonic waves with frequencies below the proton gyrofrequency have been studied by many researchers (e.g., Chen et al., 2018; Deng, Yuan, Huang, et al., 2022; K. Liu et al., 2020; X. Liu et al., 2022; Min et al., 2017; Usanova et al., 2016, 2018; Xue, Yuan, Yu, Deng, & Huang, 2022; Yao et al., 2020; Yu et al., 2017, 2021; Zhu & Chen, 2019). Some of these harmonics are considered to be oxygen ion harmonic waves which have the same spectral and propagation properties as EMIC waves, i.e., transverse electromagnetic waves and propagation with a direction almost parallel to the background magnetic field (Usanova et al., 2016, 2018).…”