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1. Role of Ducting in Relativistic Electron Loss by Whistler‐Mode Wave Scattering

   https://doi.org/10.1029/2021JA029851  

   Artemyev, A. V. ; Demekhov, A. G. ; Zhang, X. ‐J. ; Angelopoulos, V. ; Mourenas, D. ; Fedorenko, Yu V. ; Maninnen, J. ; Tsai, E. ; Wilkins, C. ; Kasahara, S. ; et al ( November 2021 , Journal of Geophysical Research: Space Physics)

   Resonant interactions of energetic electrons with electromagnetic whistler‐mode waves (whistlers) contribute significantly to the dynamics of electron fluxes in Earth's outer radiation belt. At low geomagnetic latitudes, these waves are very effective in pitch angle scattering and precipitation into the ionosphere of low equatorial pitch angle, tens of keV electrons and acceleration of high equatorial pitch angle electrons to relativistic energies. Relativistic (hundreds of keV), electrons may also be precipitated by resonant interaction with whistlers, but this requires waves propagating quasi‐parallel without significant intensity decrease to high latitudes where they can resonate with higher energy low equatorial pitch angle electrons than at the equator. Wave propagation away from the equatorial source region in a non‐uniform magnetic field leads to ray divergence from the originally field‐aligned direction and efficient wave damping by Landau resonance with suprathermal electrons, reducing the wave ability to scatter electrons at high latitudes. However, wave propagation can become ducted along field‐aligned density peaks (ducts), preventing ray divergence and wave damping. Such ducting may therefore result in significant relativistic electron precipitation. We present evidence that ducted whistlers efficiently precipitate relativistic electrons. We employ simultaneous near‐equatorial and ground‐based measurements of whistlers and low‐altitude electron precipitation measurements by ELFIN CubeSat. We show that ducted waves (appearing on the ground) efficiently scatter relativistic electrons into the loss cone, contrary to non‐ducted waves (absent on the ground) precipitating onlykeV electrons. Our results indicate that ducted whistlers may be quite significant for relativistic electron losses; they should be further studied statistically and possibly incorporated in radiation belt models.

    

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2. Ducted Chorus Waves Cause Sub‐Relativistic and Relativistic Electron Microbursts

   https://doi.org/10.1029/2021GL097559  

   Chen, Lunjin ; Zhang, Xiao‐Jia ; Artemyev, Anton ; Angelopoulos, Vassilis ; Tsai, Ethan ; Wilkins, Colin ; Horne, Richard B. ( March 2022 , Geophysical Research Letters)

   During magnetospheric storms, radiation belt electrons are produced and then removed by collisions with the lower atmosphere on varying timescales. An efficient loss process is microbursts, strong, transient precipitation of electrons over a wide energy range, from tens of keV to sub‐relativistic and relativistic energies (100s keV and above). However, the detailed generation mechanism of microbursts, especially over sub‐relativistic and relativistic energies, remains unknown. Here, we show that these energetic electron microbursts may be caused by ducted whistler‐mode lower‐band chorus waves. Using observations of equatorial chorus waves nearby low‐altitude precipitation as well as data‐driven simulations, we demonstrate that the observed microbursts are the result of resonant interaction of electrons with ducted chorus waves rather than nonducted ones. Revealing the physical mechanism behind the microbursts advances our understanding of radiation belt dynamics and its impact on the lower atmosphere and space weather.

    

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3. Energetic Electron Precipitation Driven by Electromagnetic Ion Cyclotron Waves from ELFIN’s Low Altitude Perspective

   https://doi.org/10.1007/s11214-023-00984-w  

   Angelopoulos, V. ; Zhang, X. -J. ; Artemyev, A. V. ; Mourenas, D. ; Tsai, E. ; Wilkins, C. ; Runov, A. ; Liu, J. ; Turner, D. L. ; Li, W. ; et al ( July 2023 , Space Science Reviews)

   We review comprehensive observations of electromagnetic ion cyclotron (EMIC) wave-driven energetic electron precipitation using data collected by the energetic electron detector on the Electron Losses and Fields InvestigatioN (ELFIN) mission, two polar-orbiting low-altitude spinning CubeSats, measuring 50-5000 keV electrons with good pitch-angle and energy resolution. EMIC wave-driven precipitation exhibits a distinct signature in energy-spectrograms of the precipitating-to-trapped flux ratio: peaks at >0.5 MeV which are abrupt (bursty) (lasting ∼17 s, or$\Delta L\sim 0.56$$\Delta L\sim 0.56$) with significant substructure (occasionally down to sub-second timescale). We attribute the bursty nature of the precipitation to the spatial extent and structuredness of the wave field at the equator. Multiple ELFIN passes over the same MLT sector allow us to study the spatial and temporal evolution of the EMIC wave - electron interaction region. Case studies employing conjugate ground-based or equatorial observations of the EMIC waves reveal that the energy of moderate and strong precipitation at ELFIN approximately agrees with theoretical expectations for cyclotron resonant interactions in a cold plasma. Using multiple years of ELFIN data uniformly distributed in local time, we assemble a statistical database of ∼50 events of strong EMIC wave-driven precipitation. Most reside at$L\sim 5-7$$L\sim 5-7$at dusk, while a smaller subset exists at$L\sim 8-12$$L\sim 8-12$at post-midnight. The energies of the peak-precipitation ratio and of the half-peak precipitation ratio (our proxy for the minimum resonance energy) exhibit an$L$$L$-shell dependence in good agreement with theoretical estimates based on prior statistical observations of EMIC wave power spectra. The precipitation ratio’s spectral shape for the most intense events has an exponential falloff away from the peak (i.e., on either side of$\sim 1.45$$\sim 1.45$MeV). It too agrees well with quasi-linear diffusion theory based on prior statistics of wave spectra. It should be noted though that this diffusive treatment likely includes effects from nonlinear resonant interactions (especially at high energies) and nonresonant effects from sharp wave packet edges (at low energies). Sub-MeV electron precipitation observed concurrently with strong EMIC wave-driven >1 MeV precipitation has a spectral shape that is consistent with efficient pitch-angle scattering down to ∼ 200-300 keV by much less intense higher frequency EMIC waves at dusk (where such waves are most frequent). At ∼100 keV, whistler-mode chorus may be implicated in concurrent precipitation. These results confirm the critical role of EMIC waves in driving relativistic electron losses. Nonlinear effects may abound and require further investigation.

    

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4. Characteristics of Electron Microburst Precipitation Based on High‐Resolution ELFIN Measurements

   https://doi.org/10.1029/2022JA030509  

   Zhang, Xiao‐Jia ; Angelopoulos, Vassilis ; Mourenas, Didier ; Artemyev, Anton ; Tsai, Ethan ; Wilkins, Colin ( April 2022 , Journal of Geophysical Research: Space Physics)

   We present statistical characteristics of electron microburst precipitation using high time‐resolution measurements from the low‐altitude Electron Losses and Fields InvestigatioN (ELFIN) CubeSats. The radial distribution of the equatorial projection of microbursts as a function of geomagnetic activity suggests that they are produced by resonant interaction with quasi‐parallel lower‐band chorus waves. ELFIN electron flux measurements provide the first statistical models of microburst energy spectra from 50 keV to 2 MeV. Microbursts with energies up to 150 keV have a relatively flat pitch‐angle spectrum. Estimates of scattering rates required to produce the observed flat spectra suggest that such precipitation signatures are due to near‐equatorial electron scattering by chorus wave packets with peak amplitudes of 0.4–0.9 nT, well above the threshold for nonlinear resonant interaction. More rare microbursts, exceeding 500 keV, are observed preferentially near dawn during disturbed periods. We interpret them as evidence of scattering by intense ducted chorus waves propagating from the equator up to middle latitudes with little attenuation.

    

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5. Energetic Electron Scattering due to Whistler Mode Chorus Waves Using Realistic Magnetic Field and Density Models in Jupiter's Magnetosphere

   https://doi.org/10.1029/2020JA027968  

   Ma, Q. ; Li, W. ; Zhang, X. ‐J. ; Bagenal, F. ( August 2020 , Journal of Geophysical Research: Space Physics)

   We evaluate energetic electron scattering in pitch angle and energy using realistic magnetic field and density models due to whistler mode chorus waves in Jupiter's magnetosphere and study their dependences on various wave and background parameters. We calculate the bounce‐averaged diffusion coefficients by considering the latitudinal variation of total electron density and ambient magnetic field intensity, using the VIP4 internal magnetic field and CAN current sheet model. The electron phase space density evolution due to chorus waves is simulated atMshell of 10, using the central wave frequency at0.1f_ceand wave amplitude of 30 pT. Under the typical values of the ratio between the plasma frequency and electron cyclotron frequency, chorus waves could cause fast pitch angle scattering loss of energetic electrons from tens to several hundred keV in several hours, and gradual acceleration of relativistic electrons at several MeV in several days. The electron pitch angle scattering at ~500 keV and the acceleration at several MeV are both enhanced using the latitudinally varying density and VIP4 + CAN magnetic field model compared to the electron evolution using the constant density and dipole magnetic field model. Our sensitivity study indicates that the electron scattering at higher energy is caused by waves at lower frequencies or in a lower‐density background plasma, and the scattering is faster for waves at smaller wave normal angles. The electron diffusion is mainly caused by waves at lower latitudes, but the waves at higher latitudes (>30°) contribute to the electron loss at higher energies (>2 MeV).

    

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