Bounce resonance scattering of radiation belt electrons by H + band EMIC waves

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Bounce resonant interactions with magnetospheric waves have been proposed as an important contributing mechanism for scattering near‐equatorially mirroring electrons by violating the second adiabatic invariant associated with the electron bounce motion along a geomagnetic field line. This study demonstrates that low‐frequency plasmaspheric hiss with significant wave power below 100 Hz can bounce resonate efficiently with radiation belt electrons. By performing quantitative calculations of pitch angle scattering rates, we show that low‐frequency hiss‐induced bounce resonant scattering of electrons has a strong dependence on equatorial pitch angle αeq. For electrons with αeq close to 90°, the timescale associated with bounce resonance scattering can be comparable to or even less than 1 h. Cyclotron and Landau resonant interactions between low‐frequency hiss and electrons are also investigated for comparisons. It is found that while the bounce and Landau resonances are responsible for the diffusive transport of near‐equatorially mirroring electrons to lower αeq, pitch angle scattering by cyclotron resonance could take over to further diffuse electrons into the atmosphere. Bounce resonance provides a more efficient pitch angle scattering mechanism of relativistic (≥1 MeV) electrons than Landau resonance due to the stronger scattering rates and broader resonance coverage of αeq, thereby demonstrating that bounce resonance scattering by low‐frequency hiss can contribute importantly to the evolution of the electron pitch angle distribution and the loss of radiation belt electrons.

Section: Numerical Resultsmentioning

confidence: 97%

“…The resonance condition for bounce resonant wave‐particle interactions can be written as follows (e.g., Cao et al, ; Roberts & Schulz, ; Shprits, , ):

f = l f_{b},

T ((), α_{eq}) = 1.3802 - 0.31987 ((), sin ((), α_{eq}) + \sqrt{sin ((), α_{eq})})

.…”

Section: Numerical Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 95%

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Bounce Resonance Scattering of Radiation Belt Electrons by Low‐Frequency Hiss: Comparison With Cyclotron and Landau Resonances

Cao

Summers

et al. 2017

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“…One concept is Landau resonance with oblique EMIC waves (Fu et al, ; Wang et al, ), which scatters near‐equatorially mirroring electrons to lower pitch angles. Additionally, although most previous works considered cyclotron resonant interactions, Cao et al () considered the bounce resonant scattering of near‐equatorially mirroring electrons by H + ‐band EMIC waves (Roberts & Schulz, ). H + ‐band EMIC waves were found to efficiently scatter electrons of >100 keV from ~90° pitch angles to more field‐aligned pitch angles, where the cyclotron resonance can further scatter electrons into the loss cone.…”

Section: Introductionmentioning

confidence: 99%

Nonlinear Scattering of 90° Pitch Angle Electrons in the Outer Radiation Belt by Large‐Amplitude EMIC Waves

Lee

Kim

Choi

2020

Electromagnetic ion cyclotron (EMIC) waves can cause relativistic electron scattering and atmospheric precipitation, primarily via cyclotron resonant interactions in the Earth's radiation belts. However, the conventional quasilinear resonance theory suggests that the cyclotron resonance condition is not satisfied for 90° pitch angle (PA) electrons, which constitute the majority of electrons in the outer radiation belt, such that scattering mainly affects low‐PA electrons. In contrast to this theory, using test particle calculations, we demonstrate that even exactly 90° PA electrons can be significantly scattered by large‐amplitude EMIC waves. The finite wave force results in the parallel transport of 90° PA electrons away from the equator, corresponding to intrinsically nonresonant scattering. This can lead to parallel velocity that meets cyclotron resonance conditions as local PA deviates from 90°. Different types of resonance are identified depending on the wave normal angle, that is, first‐ and second‐order resonances for parallel and oblique waves, respectively.

“…Radiation belt electrons can undergo pitch angle scattering through interactions with various wave modes, including whistler mode chorus, plasmaspheric hiss, magnetosonic waves, and electromagnetic ion cyclotron waves (e.g., Cao, Ni, Summers, Bortnik, et al, 2017;Ni et al, 2013Ni et al, , 2015Ni et al, , 2017Ni, Zou, et al, 2018;Summers et al, 2007aSummers et al, , 2007bThorne, 2010). In particular, as an important physical process in the inner magnetosphere, plasmaspheric hiss is a typically structureless, broadband, and naturally occurring whistler mode emission generally confined within the dense plasmasphere and high-density plasmaspheric plumes (e.g., Carpenter et al, 1993;Chappell, 1974;Laakso et al, 2015;Su et al, 2018;Thorne, 2010), while the fine structure of plasmaspheric hiss has been observed by Van Allen Probes, as recently reported by Summers et al (2014).…”

Section: Introductionmentioning

confidence: 99%

Electron Scattering by Plasmaspheric Hiss in a Nightside Plume

Zhang

et al. 2018

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Plasmaspheric hiss is known to play an important role in radiation belt electron dynamics in high plasma density regions. We present observations of two crossings of a plasmaspheric plume by the Van Allen Probes on 26 December 2012, which occurred unusually at the post‐midnight‐to‐dawn sector between L ~ 4–6 during a geomagnetically quiet period. This plume exhibited pronounced electron densities higher than those of the average plume level. Moderate hiss emissions accompanied the two plume crossings with the peak power at about 100 Hz. Quantification of quasi‐linear bounce‐averaged electron scattering rates by hiss in the plume demonstrates that the waves are efficient to pitch angle scatter ~10–100 keV electrons at rates up to ~10−4 s−1 near the loss cone but become gradually insignificant to scatter the higher energy electron population. The resultant timescales of electron loss due to hiss in the nightside plume vary largely with electron kinetic energy over 3 orders of magnitude, that is, from several hours for tens of keV electrons to a few days for hundreds of keV electrons to well above 100 days for >1 MeV electrons. Changing slightly with L‐shell and the multiquartile profile of hiss spectral intensity, these electron loss timescales suggest that hiss emissions in the nightside plume act as a viable candidate for the fast loss of the ≲100 keV electrons and the slow decay of higher energy electrons.