Fine Harmonic Structure of Equatorial Noise with a Quasiperiodic Modulation

Abstract

Equatorial noise emissions (fast magnetosonic waves) are electromagnetic waves observed routinely in the equatorial region of the inner magnetosphere. They propagate with wave vectors nearly perpendicular to the ambient magnetic field; that is, they are limited to frequencies below the lower hybrid frequency. The waves are generated by instabilities of ring-like proton distribution functions, which result in their fine harmonic structure with intensity maxima close to harmonics of the proton cyclotron frequency in the source region. Although most equatorial noise emissions are continuous in time, some events exhibit a clear quasiperiodic time modulation of the wave intensity, with typical modulation periods on the order of minutes. We analyze 72 such events (17 observed by the Cluster spacecraft, 55 observed by the Van Allen Probes spacecraft) for which high-resolution data were available. The analysis of the observed harmonic structure allows us to determine source radial distances of the events. It is found that the calculated source radial distances are generally close to the radial distances where the events were observed. The harmonic numbers where the events are generated range between about 12 and 30. Two events for which the spacecraft passed through the generation region were identified and analyzed. No simultaneous ultra-low-frequency magnetic field pulsations and no periodic plasma number density variations were observed. Although the in situ measured proton distribution functions were shown to be responsible for the wave growth, an insufficient resolution of the particle instruments prevented us from detecting a quasiperiodic modulation possibly present in the particle data.

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

“…While these compound rising‐tone magnetosonic waves have similar temporal scales to elementary rising‐tone waves, their sweep rate is about 70 Hz/min, much higher than that of elementary ones. Our detailed study shows that these periodic occurring of compounding rising‐tone waves are not modulated by magnetic field, plasma density, or energetic proton distribution, consistent with previous studies (e.g., Boardsen et al., 2014; Fu, Cao, Zhima, et al., 2014; Němec et al., 2020).…”

“…Burst mode waveform measured by Cluster satellites shows rising-tone magnetosonic waves that consist of multiple harmonics (Němec et al, 2015), and a case study suggests that the sweep rate maximizes at the equator (Walker et al, 2016). Van Allen Probe also observed the harmonic structure of rising-tone magnetosonic waves (Němec et al, 2020). These works investigated the magnetic field, plasma density and proton distribution variation in association with rising-tone magnetosonic waves, and no evidence of a modulation source has been clearly identified.…”

Modulation of Magnetosonic Waves by Background Plasma Density in a Dipole Magnetic Field: 2‐D PIC Simulation

“…Modulation of electromagnetic waves by the background plasma density is a common phenomenon in the magnetosphere, such as the whistlers (W. Li et al., 2011), the EMIC (S. Liu et al., 2019), and the MS waves (Němec et al., 2020; Yuan et al., 2017). Different from the whistle and EMIC waves that propagate parallel to the background magnetic field, the MS waves propagate nearly perpendicular to the background magnetic field.…”

The Electric and Magnetic Fields Instrument Suite and Integrated Science (EMFISIS): Science, Data, and Usage Best Practices

“…Several excitation mechanisms have been studied by Min et al (2018), and the effects of hot protons (Liu et al 2018b), substorm injections (Su et al 2017), and resulting favourable conditions for their excitation have been examined by Kim and Shprits (2018). Interestingly, the MS wave power tends to exhibit fine harmonic structure that can have rising tones, or be quasiperiodically modulated (Boardsen et al 2014;Li et al 2017a;Němec et al 2018Němec et al , 2020Liu et al 2018b). This modulation has been shown to be related to a number of factors that include ULF waves (Zhu et al 2019a;Liu et al 2019), solar wind pressure variations, and the resulting compression and expansion of the magnetosphere (Li et al 2017b).…”