Fine Structure of Magnetospheric Magnetosonic Waves: 1. Elementary Rising‐Tone Emissions Within Individual Harmonic

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. We studied compound rising‐tone magnetosonic waves, and found that they typically consist of multiple harmonics in the source region, and may gradually become continuous in frequency as they propagate away from source. Both elementary and compound rising‐tone magnetosonic waves last for ∼1 min which is close to the bounce period of the ring proton distribution, but their relation is not fully understood. Plain Language Summary Naturally occurring magnetosonic waves are a kind of electromagnetic wave in the Earth's magnetized space. Their frequency spectra typically exhibit a harmonic structure. Sometimes they exhibit a rising‐tone feature, that is, the wave starts at a low pitch and gradually increases its frequency to a high pitch. Using the high‐resolution data measured by Van Allen Probe satellites, the present study uncovers that each harmonic of a magnetosonic wave may have a fine structure: they consist of a series of elementary rising‐tone emissions. The traditionally known rising‐tone magnetosonic waves consist of multiple harmonics, and we name them “compound rising‐tone magnetosonic waves”. The compound rising‐tone waves may gradually become continuous in spectrum as they propagate away from source. These findings give new insights to the generation of magnetosonic waves and their impact on space environment. Key Points Each harmonic of a magnetosonic wave may consist of a series of elementary rising‐tone emissions The frequency sweep rate of elementary magnetosonic waves is likely proportional to the frequency Compound rising‐tone magnetosonic waves consist of harmonics at the source, and gradually develop a continuous spectrum during propagation