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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. 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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3. Multi‐Event Analysis of the Correlation Between Chorus Waves and Electron Butterfly Distribution Using Van Allen Probes Observation

   https://doi.org/10.1029/2022JA030806  

   Peng, Y. ; Ma, Q. ; Li, W. ; Gan, L. ; Shen, X. ‐C. ( December 2022 , Journal of Geophysical Research: Space Physics)

   In this study, using Van Allen Probes observations we identify 81 events of electron flux bursts with butterfly pitch angle distributions for tens of keV electrons with close correlations with chorus wave bursts in the Earth's magnetosphere. We use the high‐rate electron flux data from Magnetic Electron Ion Spectrometer available during 2013–2019 and the simultaneous whistler‐mode wave measurements from Electric and Magnetic Field Instrument Suite and Integrated Science to identify the correlated events. The events are categorized into 67 upper‐band chorus (0.5–0.8f_ce) dominated events and 14 other events where lower‐band chorus (0.05–0.5f_ce) has modest or strong amplitudes (f_cerepresents electron cyclotron frequency). Each electron flux burst correlated with chorus has a short timescale of ∼1 min or less, suggesting potential nonlinear effects. The statistical distribution of selected electron burst events tends to occur in the post‐midnight sector atL > 5 under disturbed geomagnetic conditions, and is associated with chorus waves with relatively strong magnetic wave amplitude and small wave normal angle. The frequency dependence of the electron flux peaks agrees with the cyclotron resonant condition, indicating the effects of chorus‐induced electron acceleration. Our study provides new insights into understanding the rapid nonlinear interactions between chorus and energetic electrons.

    

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4. 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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5. Global Model of Whistler Mode Chorus in the Near‐Equatorial Region (| λ m |<  18°)

   https://doi.org/10.1029/2020GL087311  

   Meredith, Nigel P. ; Horne, Richard B. ; Shen, Xiao‐Chen ; Li, Wen ; Bortnik, Jacob ( June 2020 , Geophysical Research Letters)

   We extend our database of whistler mode chorus, based on data from seven satellites, by including ∼3 years of data from Radiation Belt Storm Probes (RBSP)‐A and RBSP‐B and an additional ∼6 years of data from Time History of Events and Macroscale Interactions during Substorms (THEMIS)‐A, THEMIS‐D, and THEMIS‐E. The new database allows us to probe the near‐equatorial region in detail, revealing new features. In the equatorial source region, |λ_m|<6°, strong wave power is most extensive in the 0.1–0.4f_cebands in the region 21–11 magnetic local time (MLT) from the plasmapause out toL^∗ = 8 and beyond, especially near dawn. At higher frequencies, in the 0.4–0.6f_cefrequency bands, strong wave power is more tightly confined, typically being restricted to the postmidnight sector in the region 4<L^∗<6. The global distribution of strong chorus wave power changes dramatically with increasing magnetic latitude, with strong chorus waves in the region 12<|λ_m|<18° predominantly observed at frequencies below 0.3f_cein the prenoon sector, in the region 5<L^∗<8.

    

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