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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. On the Nature of Intense Sub‐Relativistic Electron Precipitation

   https://doi.org/10.1029/2022JA030571  

   Artemyev, A. V. ; Zhang, X. ‐J. ; Zou, Y. ; Mourenas, D. ; Angelopoulos, V. ; Vainchtein, D. ; Tsai, E. ; Wilkins, C. ( June 2022 , Journal of Geophysical Research: Space Physics)

   Energetic electron precipitation into Earth's atmosphere is an important process for radiation belt dynamics and magnetosphere‐ionosphere coupling. The most intense form of such precipitation is microbursts—short‐lived bursts of precipitating fluxes detected on low‐altitude spacecraft. Due to the wide energy range of microbursts (from sub‐relativistic to relativistic energies) and their transient nature, they are thought to be predominantly associated with energetic electron scattering into the loss cone via cyclotron resonance with field‐aligned intense whistler‐mode chorus waves. In this study, we show that intense sub‐relativistic microbursts may be generated via electron nonlinear Landau resonance with very oblique whistler‐mode waves. We combine a theoretical model of nonlinear Landau resonance, equatorial observations of intense very oblique whistler‐mode waves, and conjugate low‐altitude observations of <200 keV electron precipitation. Based on model comparison with observed precipitation, we suggest that such sub‐relativistic microbursts occur by plasma sheet (0.1 − 10 keV) electron trapping in nonlinear Landau resonance, resulting in acceleration to ≲200 keV energies and simultaneous transport into the loss cone. The proposed scenario of intense sub‐relativistic (≲200 keV) microbursts demonstrates the importance of very oblique whistler‐mode waves for radiation belt dynamics.

    

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4. Temporal Scales of Electron Precipitation Driven by Whistler‐Mode Waves

   https://doi.org/10.1029/2022JA031087  

   Zhang, Xiao‐Jia ; Angelopoulos, Vassilis ; Artemyev, Anton ; Mourenas, Didier ; Agapitov, Oleksiy ; Tsai, Ethan ; Wilkins, Colin ( January 2023 , Journal of Geophysical Research: Space Physics)

   Electron resonant scattering by whistler‐mode waves is one of the most important mechanisms responsible for electron precipitation to the Earth's atmosphere. The temporal and spatial scales of such precipitation are dictated by properties of their wave source and background plasma characteristics, which control the efficiency of electron resonant scattering. We investigate these scales with measurements from the two low‐altitude Electron Losses and Fields Investigation (ELFIN) CubeSats that move practically along the same orbit, with along‐track separations ranging from seconds to tens of minutes. Conjunctions with the equatorial THEMIS mission are also used to aid our interpretation. We compare the variations in energetic electron precipitation at the sameL‐shells but on successive data collection orbit tracks by the two ELFIN satellites. Variations seen at the smallest inter‐satellite separations, those of less than a few seconds, are likely associated with whistler‐mode chorus elements or with the scale of chorus wave packets (0.1–1 s in time and ∼100 km in space at the equator). Variations between precipitationL‐shell profiles at intermediate inter‐satellite separations, a few seconds to about 1 min, are likely associated with whistler‐mode wave power modulations by ultra‐low frequency waves, that is, with the wave source region (from a few to tens of seconds to a few minutes in time and ∼1,000 km in space at the equator). During these two types of variations, consecutive crossings are associated with precipitationL‐shell profiles very similar to each other. Therefore the spatial and temporal variations at those scales do not change the net electron loss from the outer radiation belt. Variations at the largest range of inter‐satellite separations, several minutes to more than 10 min, are likely associated with mesoscale equatorial plasma structures that are affected by convection (at minutes to tens of minutes temporal variations and ≈[10³, 10⁴] km spatial scales). The latter type of variations results in appreciable changes in the precipitationL‐shell profiles and can significantly modify the net electron losses during successive tracks. Thus, such mesoscale variations should be included in simulations of the radiation belt dynamics.

    

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5. Relativistic Electron Precipitation Driven by Nonlinear Resonance With Whistler‐Mode Waves

   https://doi.org/10.1029/2022JA030338  

   Tsai, Ethan ; Artemyev, Anton ; Zhang, Xiao‐Jia ; Angelopoulos, Vassilis ( May 2022 , Journal of Geophysical Research: Space Physics)

   Electron losses from the outer radiation belt are typically attributed to resonant electron scattering by whistler‐mode waves. Although the quasi‐linear diffusive regime of such scattering is well understood, the observed waves are often quite intense and in the nonlinear regime of resonant wave‐particle interaction. Such nonlinear resonant interactions are still being actively studied due to their potential for driving fast precipitation. However, direct observations of nonlinear resonance of whistler‐mode waves with electron distributions are scarce. Here, we present evidence for such resonance with high‐resolution electron energy and pitch angle spectra acquired at low‐altitudes by the dual Electron Losses and Fields INvestgation (ELFIN) CubeSats combined with conjugate measurements of equatorial plasma parameters, wave properties, and electron energy spectra by the Time History of Events and Macroscale Interactions during Substorms and Magnetospheric MultiScale missions. ELFIN has obtained numerous conjunction events exhibiting whistler wave driven precipitation; in this study, we present two such events which epitomize signatures of nonlinear resonant scattering. A test particle simulation of electron interactions with intense whistler‐mode waves prescribed at the equator is employed to directly compare modeled precipitation spectra with ELFIN observations. We show that the observed precipitating spectra match expectations to within observational uncertainties of wave amplitude for reasonable assumptions of wave power distribution along the magnetic field line. These results indicate the importance of nonlinear resonant effects when describing intense precipitation patterns of energetic electrons and open the possibility of remotely investigating equatorial wave properties using just properties of precipitation energy and pitch angle spectra.

    

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