Ultra-relativistic (multi-MeV) electrons in Earth's outer Van Allen radiation belt are removed primarily by pitch-angle scattering into the atmospheric loss cone, by magnetopause shadowing, or by outward radial diffusion. A 2016 event study (Shprits et al., Nat. Commun. 7, 12883) of the 17 January 2013 dropout argued that electromagnetic ion cyclotron (EMIC) waves provide the dominant loss mechanism at ultra-relativistic energies, but rested on assumed wave geometry (waves confined to 5% of the drift orbit above a modeled plasmapause), GOES-15-derived spectra, and quasi-linear diffusion coefficients. Direct coincident observations (Van Allen Probes, and the low-Earth-orbit missions FIREBIRD-II, ELFIN, and CIRBE/REPTile-2) now confirm EMIC-driven relativistic precipitation, and statistical/assimilation studies establish that EMIC loss dominates at lower L* and the highest energies while magnetopause shadowing dominates at higher L*; however, the quantitative storm-level partition, validated MLT/L-dependent EMIC climatology, and nonlinear-corrected diffusion parameterizations remain unfinished, with 2025-2026 papers still describing the precise scattering effects as debated. The problem is open.

Background

Earth's radiation belts are toroidal regions of trapped energetic electrons and ions centered on field lines at roughly 1.2-6 Earth radii, produced by the near-dipolar geomagnetic field. Trapped electrons execute three periodic motions (gyration, bounce between magnetic mirror points, and drift around Earth) characterized by three adiabatic invariants; the third invariant is conventionally labeled by L∗L^*, the drift-shell radius in Earth radii for a dipole field. Electrons are lost when they are scattered in pitch angle (the angle between the particle velocity and the magnetic field) into the loss cone, the range of pitch angles that intersects the atmosphere, where they precipitate; they can also be lost by magnetopause shadowing, in which outward radial transport carries them across the magnetopause boundary. Electromagnetic ion cyclotron (EMIC) waves are plasma waves at frequencies below the proton gyrofrequency, generated by anisotropic ring-current protons; through cyclotron resonance they can scatter electrons with energies of a few MeV and above into the loss cone. The minimum resonant energy for this interaction depends steeply on the ratio of plasma frequency to electron gyrofrequency, and therefore on the cold-plasma density. Quantitative modeling treats this scattering as quasi-linear pitch-angle diffusion, governed by diffusion coefficients computed from observed wave spectra, plasma density, and the background magnetic field model. EMIC waves occur preferentially in high-density regions at the plasmapause (the outer boundary of the cold plasmasphere) and in plumes of plasmapheric plasma drawn outward during storms, and magnetic local time (MLT) sectors are usually identified in hours around dusk and the dayside. The 17 January 2013 event observed by the Van Allen Probes mission (two spacecraft carrying the Magnetic Electron Ion Spectrometer (MagEIS) and Relativistic Electron Proton Telescope (REPT) electron instruments) showed relativistic electrons enhanced while ultra-relativistic electrons were depleted, with sharply narrowed pitch-angle distributions that Shprits et al. (2016) identified as EMIC signatures; 1D/3D diffusion simulations with the Versatile Electron Radiation Belt (VERB) code reproduced the dropout only when EMIC waves were included, with EMIC activity assumed to occupy 5% of the electron drift orbit above a plasmapause location from a plasmapause test-particle (PTP) model, and wave spectra taken from the GOES 15 geostationary weather spacecraft (part of the Geostationary Operational Environmental Satellite series) and Finnish ground magnetometers. The study concluded that EMIC waves provide the dominant loss mechanism at ultra-relativistic energies and may impose an upper energy limit on the trapped population, remarking that similar processes may operate at the outer planets and exoplanets. The claim was event-specific, and the study's assumptions about wave spatial distribution, MLT extent, and diffusion coefficients were unvalidated, leaving the prevalence and magnitude of EMIC-driven ultra-relativistic loss unquantified. A related but distinct question, the 'impenetrable barrier' at the inner edge of the ultra-relativistic belt, was later explained by Ozeke et al. (2018) as a consequence of ultra-low-frequency wave radial diffusion rather than fast EMIC loss, so it should not be conflated with the loss-partition problem here.

Problem Statement

Determine, quantitatively and observationally, whether electromagnetic ion cyclotron (EMIC) waves are the dominant sink of ultra-relativistic (multi-MeV) electrons in Earth's outer radiation belt across geomagnetic storm conditions - rather than only under particular plasmaspheric-plume and seed-population configurations (the seed population being the lower-energy source electrons that storms accelerate to relativistic energies) as in the 17 January 2013 event - and thereby establish the prevalence, magnitude, and spatial/energy structure of EMIC-driven ultra-relativistic loss. Here 'dominant' means the largest single contributor to the ultra-relativistic electron loss rate among the three loss channels named in component (i) below (EMIC-wave scattering, magnetopause shadowing, and whistler-mode wave scattering), and does not require a majority of the total loss. The following components are jointly in service of this single objective: (i) establish the storm-level partition of ultra-relativistic electron loss among EMIC-wave scattering, magnetopause shadowing, and scattering by whistler-mode waves (the branch of plasma waves, including chorus and plasmaspheric hiss emissions, that resonates with electrons at generally lower energies than EMIC waves do) as a function of energy, L∗L^*, and geomagnetic driving; (ii) replace the assumed EMIC geometry of the source study (activity confined to 5% of the drift orbit above a modeled plasmapause) with observationally validated magnetic-local-time, L∗L^*, and activity-dependent wave occurrence, spatial extent, and amplitude climatology, and quantify the sensitivity of computed loss to that distribution; (iii) deliver diffusion-coefficient parameterizations that propagate the uncertainties of sparse wave measurements, plasma density and composition, hot-plasma dispersion, and nonlinear scattering effects beyond quasi-linear theory (a post-2016 development not treated in the source study); and (iv) demonstrate closure against event-resolved, pitch-angle-resolved ultra-relativistic electron observations coincident with in-situ and ground-based EMIC wave measurements. As a separable extension suggested by the source study's Discussion remark that similar processes may occur on the outer planets and exoplanets, assess whether the same EMIC-induced upper-energy limitation on trapped electrons applies to other planetary magnetospheres (for example Saturn) and exoplanetary environments where plasma densities and wave activity differ. Components (i)-(iv) are jointly required for this determination and none is independently sufficient: the objective is resolved only when a single, self-consistent determination of storm-level EMIC dominance is supported by all four together, so addressing any proper subset of them does not resolve the task. The planetary extension described in the preceding sentence is optional and does not affect resolution of the terrestrial objective.

References

  1. Shprits, Y. Y., Drozdov, A. Y., Spasojevic, M., Kellerman, A. C., Usanova, M. E., Engebretson, M. J., Agapitov, O. V., Zhelavskaya, I. S., Raita, T. J. P., Spence, H. E., Baker, D. N., Zhu, H., Aseev, N. A. (2016). Wave-induced loss of ultra-relativistic electrons in the Van Allen radiation belts. Nature Communications 7, 12883. doi:10.1038/ncomms12883. https://doi.org/10.1038/ncomms12883
  2. Aseev, N. A., Shprits, Y. Y., Drozdov, A. Y., Kellerman, A. C., Usanova, M. E., Wang, D., Zhelavskaya, I. S. (2017). Signatures of Ultrarelativistic Electron Loss in the Heart of the Outer Radiation Belt Measured by Van Allen Probes. Journal of Geophysical Research: Space Physics 122. doi:10.1002/2017JA024485. https://doi.org/10.1002/2017JA024485
  3. Xiang, Z., Tu, W., Li, X., Ni, B., Morley, S. K., Baker, D. N. (2017). Understanding the Mechanisms of Radiation Belt Dropouts Observed by Van Allen Probes. Journal of Geophysical Research: Space Physics 122, 9858-9879. doi:10.1002/2017JA024487. https://doi.org/10.1002/2017JA024487
  4. Su, Z., Gao, Z., Zheng, H., Wang, Y., Wang, S., Spence, H. E., Reeves, G. D., Baker, D. N., Wygant, J. R. (2017). Rapid Loss of Radiation Belt Relativistic Electrons by EMIC Waves. Journal of Geophysical Research: Space Physics 122, 9880-9897. doi:10.1002/2017JA024169. https://doi.org/10.1002/2017JA024169
  5. Drozdov, A. Y., Shprits, Y. Y., Usanova, M. E., Aseev, N. A., Kellerman, A. C., Zhu, H. (2017). EMIC wave parameterization in the long-term VERB code simulation. Journal of Geophysical Research: Space Physics 122, 8488-8501. doi:10.1002/2017JA024389. https://doi.org/10.1002/2017JA024389
  6. Blum, L. W., Bonnell, J. W., Agapitov, O., Paulson, K., Kletzing, C. (2017). EMIC wave scale size in the inner magnetosphere: Observations from the dual Van Allen Probes. Geophysical Research Letters 44, 1227-1233. doi:10.1002/2016GL072316. https://doi.org/10.1002/2016GL072316
  7. Cao, X., Shprits, Y. Y., Ni, B., Zhelavskaya, I. S. (2017). Scattering of Ultra-relativistic Electrons in the Van Allen Radiation Belts Accounting for Hot Plasma Effects. Scientific Reports 7, 17719. doi:10.1038/s41598-017-17739-7. https://doi.org/10.1038/s41598-017-17739-7
  8. Xiang, Z., Tu, W., Ni, B., Henderson, M. G., Cao, X. (2018). A Statistical Survey of Radiation Belt Dropouts Observed by Van Allen Probes. Geophysical Research Letters 45, 8035-8043. doi:10.1029/2018GL078907. https://doi.org/10.1029/2018GL078907
  9. Ni, B., Cao, X., Shprits, Y. Y., Summers, D., Gu, X., Fu, S., Lou, Y. (2018). Hot Plasma Effects on the Cyclotron-Resonant Pitch-Angle Scattering Rates of Radiation Belt Electrons Due to EMIC Waves. Geophysical Research Letters 45, 21-30. doi:10.1002/2017GL076028. https://doi.org/10.1002/2017GL076028
  10. Ozeke, L. G., Mann, I. R., Murphy, K. R., Degeling, A. W., Claudepierre, S. G., Spence, H. E. (2018). Explaining the apparent impenetrable barrier to ultra-relativistic electrons in the outer Van Allen belt. Nature Communications 9, 1844. doi:10.1038/s41467-018-04162-3. https://doi.org/10.1038/s41467-018-04162-3
  11. Zhu, H., Chen, L., Claudepierre, S. G., Zheng, L. (2020). Direct Evidence of the Pitch Angle Scattering of Relativistic Electrons Induced by EMIC Waves. Geophysical Research Letters 47, e2019GL085637. doi:10.1029/2019GL085637. https://doi.org/10.1029/2019GL085637
  12. Ross, J. P. J., Glauert, S. A., Horne, R. B., Watt, C. E., Meredith, N. P., Woodfield, E. E. (2020). A New Approach to Constructing Models of Electron Diffusion by EMIC Waves in the Radiation Belts. Geophysical Research Letters 47, e2020GL088976. doi:10.1029/2020GL088976. https://doi.org/10.1029/2020GL088976
  13. Shekhar, S., Millan, R. M., Woodger, L. A., Qin, M. (2020). Quantification of the Atmospheric Relativistic Electron Precipitation on 17 January 2013. Journal of Geophysical Research: Space Physics 125, e2020JA028014. doi:10.1029/2020JA028014. https://doi.org/10.1029/2020JA028014
  14. Cervantes, S., Shprits, Y. Y., Aseev, N. A., Allison, H. J. (2020). Quantifying the Effects of EMIC Wave Scattering and Magnetopause Shadowing in the Outer Electron Radiation Belt by Means of Data Assimilation. Journal of Geophysical Research: Space Physics 125, e2020JA028208. doi:10.1029/2020JA028208. https://doi.org/10.1029/2020JA028208
  15. Capannolo, L., Li, W., Spence, H., Johnson, A. T., Shumko, M., Sample, J., Klumpar, D. (2021). Energetic Electron Precipitation Observed by FIREBIRD-II Potentially Driven by EMIC Waves: Location, Extent, and Energy Range From a Multievent Analysis. Geophysical Research Letters 48, e2020GL091564. doi:10.1029/2020GL091564. https://doi.org/10.1029/2020GL091564
  16. Ross, J. P. J., Glauert, S. A., Horne, R. B., Watt, C. E. J., Meredith, N. P. (2021). On the Variability of EMIC Waves and the Consequences for the Relativistic Electron Radiation Belt Population. Journal of Geophysical Research: Space Physics 126, e2021JA029754. doi:10.1029/2021JA029754. https://doi.org/10.1029/2021JA029754
  17. Grach, V. S., Artemyev, A. V., Demekhov, A. G., Zhang, X.-J., Bortnik, J., Angelopoulos, V., Nakamura, R., Tsai, E., Wilkins, C., Roberts, O. W. (2022). Relativistic Electron Precipitation by EMIC Waves: Importance of Nonlinear Resonant Effects. Geophysical Research Letters 49, e2022GL099994. doi:10.1029/2022GL099994. https://doi.org/10.1029/2022GL099994
  18. Capannolo, L., Li, W., Ma, Q., Qin, M., Shen, X.-C., Angelopoulos, V., Artemyev, A., Zhang, X.-J., Hanzelka, M. (2023). Electron Precipitation Observed by ELFIN Using Proton Precipitation as a Proxy for Electromagnetic Ion Cyclotron (EMIC) Waves. Geophysical Research Letters 50, e2023GL103519. doi:10.1029/2023GL103519. https://doi.org/10.1029/2023GL103519
  19. Cao, X., Lu, P., Ni, B., Summers, D., Shprits, Y. Y., Long, M., Wang, X. (2023). Resonant Scattering of Radiation Belt Electrons at Saturn by Ion Cyclotron Waves. Geophysical Research Letters 50, e2022GL102394. doi:10.1029/2022GL102394. https://doi.org/10.1029/2022GL102394
  20. Inglis, A. R., Murphy, K. R., Halford, A. (2024). A Survey of EMIC Waves in Van Allen Probe Data. Journal of Geophysical Research: Space Physics 129, e2024JA032666. doi:10.1029/2024JA032666. https://doi.org/10.1029/2024JA032666
  21. Bashir, M. F., Artemyev, A., Zhang, X.-J., Angelopoulos, V., Tsai, E., Wilkins, C. (2024). Observations of Relativistic Electron Precipitation Due To Combined Scattering of Whistler-Mode and EMIC Waves. Journal of Geophysical Research: Space Physics 129, e2024JA032432. doi:10.1029/2024JA032432. https://doi.org/10.1029/2024JA032432
  22. Xiang, Z., Li, X., Baker, D. N., Sarris, T., Zhao, H., O'Brien, D., Mei, Y., Temerin, M. A. (2025). Fine Structure of Relativistic Electron Precipitation Driven by EMIC Waves: CIRBE/REPTile-2 Measurements and Physical Implications. Geophysical Research Letters 52, e2025GL117988. doi:10.1029/2025GL117988. https://doi.org/10.1029/2025GL117988
  23. Huang, J., Lyu, X., Tu, W., Ma, Q., Selesnick, R., Liu, X., Chen, L. (2025). Modeling Relativistic Electron Dropout in the Outer Radiation Belt During the 31 December 2016 Storm. Journal of Geophysical Research: Space Physics 130, e2025JA033965. doi:10.1029/2025JA033965. https://doi.org/10.1029/2025JA033965
  24. Xiang, Z., Li, X., Baker, D. N., O'Brien, D., Mei, Y., Zhang, Z., Sarris, T., Temerin, M. A. (2026). Sub-MeV Electron Precipitation Driven by EMIC Waves: New Insights From CIRBE/REPTile-2 Measurements. Geophysical Research Letters 53, e2026GL123209. doi:10.1029/2026GL123209. https://doi.org/10.1029/2026GL123209