First Observation of Mini Harmonic Structure in Magnetosonic Waves

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.