Fine structures of magnetospheric magnetosonic waves: elementary rising-tone emissions and mini harmonics

The present study uncovers the fine structures of magnetosonic waves by investigating the EFW waveforms measured by Van Allen Probes. We show that each harmonic of the magnetosonic wave may consist of a series of elementary rising-tone emissions, implying a nonlinear mechanism for the wave generation. By investigating an elementary rising-tone magnetosonic wave that spans a wide frequency range, we show that the frequency sweep rate is likely proportional to the wave frequency. Furthermore, we reveal that each elementary rising-tone magnetosonic waves consist of multiple mini-harmonics spaced at O+ gyrofrequency. We reveal that O+ ions can suppress the generation of magnetosonic waves at multiples of O+ gyrofrequency, resulting in the mini-harmonic structure. The commonly observed mini-harmonics indicate an energy transfer between different ion species.