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

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Electromagnetic ion cyclotron waves have long been recognized to play a crucial role in the dynamic loss of ring current protons. While the field‐aligned propagation approximation of electromagnetic ion cyclotron waves was widely used to quantify the scattering loss of ring current protons, in this study, we find that the wave normal distribution strongly affects the pitch angle scattering efficiency of protons. Increase of peak normal angle or angular width can considerably reduce the scattering rates of ≤10 keV protons. For >10 keV protons, the field‐aligned propagation approximation results in a pronounced underestimate of the scattering of intermediate equatorial pitch angle protons and overestimates the scattering of high equatorial pitch angle protons by orders of magnitude. Our results suggest that the wave normal distribution of electromagnetic ion cyclotron waves plays an important role in the pitch angle evolution and scattering loss of ring current protons and should be incorporated in future global modeling of ring current dynamics.

Section: Introductionmentioning

confidence: 99%

Sensitivity of EMIC Wave‐Driven Scattering Loss of Ring Current Protons to Wave Normal Angle Distribution

Cao

Summers

et al. 2019

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“…It is shown that low‐frequency hiss could bounce resonate efficiently with near‐equatorially mirroring electrons at energies ranging from 10 keV to 10 MeV. The scattering efficiency at L = 5 is found to be much stronger than that at L = 4 and 4.5, which is mainly due to the stronger hiss wave spectral intensities (as shown in Figure ) and lower geomagnetic field intensity at L = 5 (Cao et al, ; Tao & Li, ). Figure illustrates that the bounce resonant pitch angle scattering rates have a pronounced dependence on the electron equatorial pitch angle α eq .…”

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},

where f is the wave frequency (in Hz), l is the bounce resonance order, f b = βc /(4 LR E T ( α eq )) is the particle bounce frequency, β = v / c , v is the velocity of particle, c is the velocity of light, L is the McIlwain L shell value, R E is the Earth's radius, and

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

.…”

Section: Numerical Resultsmentioning

confidence: 99%

“…It has been proposed by Shprits () that magnetosonic and H + band electromagnetic ion cyclotron (EMIC) waves, which have frequencies comparable to the electron bounce frequency or its harmonics, have the potential to bounce resonate with near‐equatorially mirroring electrons by violating the second invariant associated with the electron bounce motion. Bounce resonant interactions with magnetosonic and H + band EMIC waves have been subsequently proved by recent studies (e.g., Chen et al, ; Cao et al, ; Li et al, ; Shprits, ; Tao & Li, ; Tao et al, ) to account for the pitch angle scattering of electrons with ~90° equatorial pitch angles. For typical plasmaspheric hiss with wave frequencies much higher than the electron bounce frequency, its bounce resonance with near‐equatorially mirroring electrons tends to be insignificant.…”

Section: Introductionmentioning

confidence: 95%

See 1 more Smart Citation

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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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.

“…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.