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First Observation of Mini Harmonic Structure in Magnetosonic Waves
Journal article   Open access   Peer reviewed

First Observation of Mini Harmonic Structure in Magnetosonic Waves

Qiushuo Wang, Jinxing Li, Jacob Bortnik, Qianli Ma, Sheng Tian, Daniel N. Baker, John Wygant, George B. Hospodarsky and Geoffrey D. Reeves
Geophysical research letters, Vol.52(11), e2025GL114908
06/16/2025
DOI: 10.1029/2025GL114908
url
https://doi.org/10.1029/2025GL114908View
Published (Version of record) Open Access

Abstract

We recently reported the finding of elementary rising‐tone emissions embedded within each harmonic of magnetosonic waves, by investigating wave electric field waveforms measured by Van Allen Probes. The present study further uncovers a new set of fine structures of magnetosonic waves, namely, each elementary rising‐tone may consist of a series of mini harmonics spaced around the O+ gyrofrequency. The measured ion distributions suggest that the proton ring distribution provides free energy to excite the waves, whilst the O+ ions suppress the wave growth around multiples of O+ gyrofrequency, resulting in the formation of mini harmonics. Further investigation suggests that the warm plasma dispersion relation, that is, the ion Bernstein mode instabilities, may contribute to the formation of mini harmonics. The mini harmonic structure implies a new mechanism of energy redistribution among ion species in space plasmas, potentially providing a new acceleration mechanism for O+ ions in the magnetosphere. Plain Language Summary Magnetosonic waves are a type of natural electromagnetic wave that helps convert and transfer energy in near‐Earth space. The most striking feature of magnetosonic waves is that they consist of a series of narrow frequency bands spaced at roughly the proton gyrofrequency. Our research, using high‐resolution data from NASA's Van Allen Probe satellites, discovered a fine structure within the waves. Each band consists of multiple mini bands, which we call the mini‐harmonics because they are spaced around the oxygen ion (O+) gyro‐frequency at which O+ ions naturally gyrate. The particle data measured by the satellite suggests that protons provide the free energy to generate these waves, while O+ ions suppress the wave growth at certain frequencies, leading to the generation of mini harmonics. Our findings provide new insights on the dynamics of O+ ions in near‐Earth space. Key Points Each harmonic of a magnetosonic wave may consist of a series of elements containing mini harmonics spaced at the O+ gyrofrequency Oxygen ions are found to suppress wave generation around multiples of O+ gyrofrequency, leading to the formation of mini harmonics The mini harmonic structure of wave power spectrum follows the ion Bernstein mode dispersion, confirming the wave absorption by O+ ions

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