Propagation and linear mode conversion of magnetosonic and electromagnetic ion cyclotron waves in the radiation belts

Equatorial noise (EN) emissions are observed inside and outside the plasmapause. EN emissions are referred to as magnetosonic mode waves. Using data from Van Allen Probes and Arase, we found conversion from EN emissions to electromagnetic ion cyclotron (EMIC) waves in the plasmasphere and in the topside ionosphere. A low‐frequency part of EN emissions becomes EMIC waves through branch splitting of EN emissions, and the mode conversion from EN to EMIC waves occurs around the frequency of M/Q = 2 (deuteron and/or alpha particles) cyclotron frequency. These processes result in plasmaspheric EMIC waves. We investigated the ion composition ratio by characteristic frequencies of EN emissions and EMIC waves and obtained ion composition ratios. We found that the maximum composition ratio of M/Q = 2 ions is ~10% below 3,000 km. The quantitative estimation of the ion composition will contribute to improving the plasma model of the deep plasmasphere and the topside ionosphere.

Section: Summary and Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

EMIC Waves Converted From Equatorial Noise Due to M/Q = 2 Ions in the Plasmasphere: Observations From Van Allen Probes and Arase

Miyoshi

Matsuda

Kurita

et al. 2019

“…The magnetosonic waves at low harmonics are nearly dispersionless (Boardsen et al, ), and their frequency‐time structures should change little during the propagation (Su et al, ). Such opposite variations in the wave frequency and the emission line spacing are difficult to be explained by the wave propagation process (Chen & Thorne, ; Horne & Miyoshi, ; Horne et al, ; Kasahara et al, ; Ma et al, ; Santolík et al, ).…”

Section: Discussionmentioning

confidence: 99%

“…Most of these waves were observed in the dayside low‐density plasmatrough following intense substorms (AE>500 nT), generally consistent with previous statistical studies of magnetosonic waves on the basis of low‐resolution data (Boardsen et al, ; Hrbáčková et al, ; Kim & Shprits, ; Ma et al, ; Meredith et al, ; Němec et al, ; Shprits et al, ). Magnetosonic waves can be destabilized by the substorm‐injected hot protons (Boardsen et al, ; Curtis & Wu, ; Gary et al, ; Gulelmi et al, ; Horne et al, ; Meredith et al, ; Yuan et al, , ) and propagate over a broad region (Chen & Thorne, ; Horne & Miyoshi, ; Kasahara et al, ; Ma et al, ; Němec et al, ; Santolík et al, ; Su et al, ), allowing the subsequent nonlinear wave‐wave interactions (Perraut et al, ). However, different from previous statistics (Yang et al, ), the occurrence of these unusual magnetosonic waves did not show any clear dependence on the geomagnetic storm activity.…”

Section: Discussionmentioning

confidence: 99%

“…This characteristic can be well explained by the local Bernstein mode instability of hot proton ring distributions (Boardsen et al, ; Curtis & Wu, ; Gary et al, ; Gulelmi et al, ; Horne et al, ; Meredith et al, ; Perraut et al, ; Yuan et al, , ). Previous ray tracing simulations have suggested that magnetosonic waves are not trapped near the source region but propagate radially and azimuthally in the low‐latitude region (Chen & Thorne, ; Horne & Miyoshi, ; Horne et al, ; Kasahara et al, ; Ma et al, ; Němec et al, ; Santolík et al, ). Because of the nearly dispersionless characteristic particularly at low harmonics (Boardsen et al, ), their frequency‐time structures change little during the propagation (Su et al, ).…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Magnetosonic Harmonic Falling and Rising Frequency Emissions Potentially Generated by Nonlinear Wave‐Wave Interactions in the Van Allen Radiation Belts

Liu

Zheng

et al. 2018

Magnetosonic waves play a potentially important role in the complex evolution of the radiation belt electrons. These waves typically appear as discrete emission lines along the proton gyrofrequency harmonics, consistent with the prediction of the local Bernstein mode instability of hot proton ring distributions. Magnetosonic waves are nearly dispersionless particularly at low harmonics and therefore have the roughly unchanged frequency time structures during the propagation. On the basis of Van Allen Probes observations, we here present the first report of magnetosonic harmonic falling and rising frequency emissions. They lasted for up to 2 hr and occurred primarily in the dayside plasmatrough following intense substorms. These harmonic emission lines were well spaced by the proton gyrofrequency but exhibited a clear falling (rising) frequency characteristic in a regime with the temporal increase (decrease) of the proton gyrofrequency harmonics. Such unexpected structures might be produced by the nonlinear interactions between the locally generated magnetosonic waves at the proton gyrofrequency harmonics and a constant frequency magnetosonic wave propagating away from the Earth.

Direct observation of generation and propagation of magnetosonic waves following substorm injection

Wang

Liu

et al. 2017

Magnetosonic whistler mode waves play an important role in the radiation belt electron dynamics. Previous theory has suggested that these waves are excited by the ring distributions of hot protons and can propagate radially and azimuthally over a broad spatial range. However, because of the challenging requirements on satellite locations and data processing techniques, this theory was difficult to validate directly. Here we present some experimental tests of the theory on the basis of Van Allen Probes observations of magnetosonic waves following substorm injections. At higher L shells with significant substorm injections, the discrete magnetosonic emission lines started approximately at the proton gyrofrequency harmonics, qualitatively consistent with the prediction of linear proton Bernstein mode instability. In the frequency‐time spectrograms, these emission lines exhibited a clear rising tone characteristic with a long duration of 15–25 min, implying the additional contribution of other undiscovered mechanisms. Nearly at the same time, the magnetosonic waves arose at lower L shells without substorm injections. The wave signals at two different locations, separated by ΔL up to 2.0 and by ΔMLT up to 4.2, displayed the consistent frequency‐time structures, strongly supporting the hypothesis about the radial and azimuthal propagation of magnetosonic waves.