Subpacket structure in strong VLF chorus rising tones: characteristics and consequences for relativistic electron acceleration

Foster, J. C.; Erickson, Philip J.; Omura, Yoshiharu

doi:10.1186/s40623-021-01467-4

Cited by 13 publications

(17 citation statements)

References 33 publications

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“…Farther away from the equator, some short packets start to appear in the first part of each long chorus element, due to a superposition of the low‐frequency start of a chorus element with the high‐frequency end of the preceding chorus element, which often shows hook‐like features. The long chorus elements obtained in this simulation are quite realistic, since very similar chorus rising tones have been observed by the Van Allen Probes around L ∼ 5–6 (Foster et al., 2021 ).…”

Section: Simulations Of Short Chorus Packet Generation: Dependence On...supporting

confidence: 69%

Short Chorus Wave Packets: Generation Within Chorus Elements, Statistics, and Consequences on Energetic Electron Precipitation

et al. 2022

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Short and intense lower‐band chorus wave packets are ubiquitous in the Earth's outer radiation belt. In this article, we perform various Vlasov hybrid simulations, with one or two triggering waves, to study the generation of short chorus packets/subpackets inside long rising tone elements. We show that the length of the generated short wave packets is consistent with a criterion of resonance non‐overlap for two independent superposed waves, and that these chorus packets have similar characteristics as in Van Allen Probes observations. We find that short wave packets are mainly formed near the middle/end of long rising tones for moderate linear growth rates, and everywhere for stronger linear growth rates. Finally, we analyze an event characterized by Time History of Events and Macroscale Interactions during Substorms spacecraft measurements of chorus rising tones near the equator and simultaneous measurements by low altitude ELFIN CubeSats of precipitating and trapped electron fluxes in the same sector. The measured precipitating electron fluxes are well recovered by test particle simulations performed using measured plasma and wave properties. We show that short chorus wave packets of moderate amplitudes (160–250 pT) essentially lead to a more diffusive‐like transport of 50–200 keV electrons toward the loss cone than long packets. In contrast, long chorus packets are found to produce important nonlinear effects via anomalous trapping, which significantly reduces electron precipitation below 150 keV, especially for higher wave amplitudes.

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Section: Simulations Of Short Chorus Packet Generation: Dependence On...supporting

confidence: 69%

Short Chorus Wave Packets: Generation Within Chorus Elements, Statistics, and Consequences on Energetic Electron Precipitation

et al. 2022

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“…As described by Zhang et al, 2020, such jumps in phase would cause de-trapping of electrons, and reduce energization caused by phase trapping. The noticeable frequency dip between first and second subpackets has been reported previously in Van Allen Probes observations (Santolik et al, 2014;Foster et al, 2021) and is in keeping with numerical simulations of chorus elements (Hanzelka et al, 2020). Foster et al, 2021 has shown the relative contributions of first and second subpackets to electron energization.…”

Section: Chorus Wave Observationssupporting

confidence: 84%

“…The presentation of our analysis of S-function effects and radiation belt electron energization has focused on a single strong chorus wave subpacket observed during the onset of the MeV radiation belt electron recovery event on 17 March 2013. We have performed similar analyses for multiple chorus elements observed at different times and L-space locations during different storms, with similar results and conclusions (Foster et al, 2021 discusses aspects of several further examples). Many of the features presented here are repeatable in nearly every observation (strong second subpacket, steeply rising df/dt near the local value of 1/ 4 f ce , dominance of cyclotron energization at subrelativistic seed electron energies).…”

Section: S Factor Dependance Of Electron Energy Gainmentioning

confidence: 62%

“…We show the smooth phase transition between initial subpackets of chorus wave elements, conducive to extended trapping and enhancement of resonant electrons. Foster et al, 2021 pointed out the steepening of the observed chorus waveform frequency sweep rate, df/dt, near the local value of 1/4 f ce for several cases and noted that it was generally seen in their observations of strong chorus elements. Here we address the effects of frequency dispersion on df/dt with a simple model based on the off-equatorial observations of df/dt and the frequency dependences of the wave group velocity, V g, and wave normal angle, WNA.…”

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confidence: 88%

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Off-equatorial effects of the nonlinear interaction of VLF chorus waves with radiation belt electrons

Foster

Erickson

2022

Front. Astron. Space Sci.

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Nonlinear processes are involved in both the growth of VLF chorus waves and the energization of radiation belt electrons trapped in the wave potential. Nonlinear theory has led to analytic formulae describing both these processes. To investigate these processes, observations from the Van Allen Probes twin spacecraft provide simultaneous in situ information on VLF chorus waves, radiation belt and injected electrons, and local plasma parameters. We combine the theoretical treatment summarized by Omura (2021) with these in situ observations to investigate the characteristics and effects of nonlinear radiation belt processes at the off-equatorial location of the spacecraft observations. We show the smooth phase transition between initial subpackets of chorus wave elements, conducive to extended trapping and enhancement of resonant electrons. The structure of the chorus wave element changes as it propagates away from the equator. Frequency dispersion due to the variation of parallel wave group velocity with frequency contributes to the chorus waveform frequency sweep rate observed at an off-equatorial location. Nonlinear damping at the local value of ½ fce progressively erodes wave amplitude at frequencies above ½ fceEQ. We examine the important dependencies of the nonlinear inhomogeneity factor on the time rate of change of the wave frequency and the field-aligned gradient of the magnetic field and discuss their implication for the energization of trapped non-relativistic and MeV electrons. The 0.5–2% energy gain we find for 3–6 MeV seed electrons indicates that prompt local acceleration of highly relativistic and ultra-relativistic radiation belt electrons can take place directly through their nonlinear interaction with an individual VLF chorus wave element.

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“…Whistler-mode waves play important roles in scattering energetic electrons into the loss cone in the magnetosphere as well as the acceleration of radiation belt electrons [e.g., Baker, 2021;Martinez-Calderon et al, 2021]. Nonlinear wave-particle interactions between whistler-mode chorus emissions and energetic electrons have been intensively studied from the point of view of the wave generation process as well as the acceleration/scattering of energetic electrons [Omura et al, 2008[Omura et al, , 2009Katoh et al, 2018;Kitahara and Katoh, 2019;Omura, 2021;Nunn, 2021;Foster et al, 2021;Liu et al, 2021]. Recent studies suggest that the periodicities of pulsating aurora can be explained by the characteristic time scale of chorus emissions [see Nishimura et al (2020) and references therein].…”

Section: Introductionmentioning

confidence: 99%

Effect of the mirror force on the collision rate due to energetic electron precipitation: Monte Carlo simulations

Katoh

Rosendahl

Ogawa

et al. 2022

Preprint

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We study the effect of the mirror force on the collision rate due to the energetic electron precipitation into the ionosphere. We develop a simulation code for the motion of energetic electrons with the mirror force to solve the variation of the pitch angle of electrons during their precipitation. In this code, a module computing the collision between precipitating energetic electrons and neutral gas using the Monte Carlo method is employed. By combining the developed modules, altitude profiles of the collision rate due to energetic electron precipitation in the keV energy range are investigated, and the effect of the mirror force acting on precipitating electrons is examined. The simulation results show that the influence of the mirror force on the altitude profile of the collision rate is significant for electrons with a high initial pitch angle, corresponding to the pitch angle close to the loss cone. The effect of the mirror force results in the broadening of the altitude profile of the collision upward due to the reflection of mirroring electrons. Simulation results for energetic electrons with kinetic energies above 100 keV show that a secondary peak near the mirror point is formed in the altitude profile of the collision rate. The formation of the secondary peak can be explained by the stagnation of electrons around the mirror point. The relatively long duration staying in neutral gas results in the increase of the collision rate around the mirror point, against the smaller collision cross-section in the higher energy range. Simulation results reveal that the maximum collision rate in the altitude range lower than 100 km becomes one order of magnitude smaller if electrons in the kinetic energy range larger than tens of keV precipitate with the pitch angle close to the loss cone. The results of the present study emphasize the importance of the mirror force for the precise modeling of ionospheric response due to the energetic electron precipitation.

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Subpacket structure in strong VLF chorus rising tones: characteristics and consequences for relativistic electron acceleration

Cited by 13 publications

References 33 publications

Short Chorus Wave Packets: Generation Within Chorus Elements, Statistics, and Consequences on Energetic Electron Precipitation

Short Chorus Wave Packets: Generation Within Chorus Elements, Statistics, and Consequences on Energetic Electron Precipitation

Off-equatorial effects of the nonlinear interaction of VLF chorus waves with radiation belt electrons

Effect of the mirror force on the collision rate due to energetic electron precipitation: Monte Carlo simulations

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