Letter | Published:

An impenetrable barrier to ultrarelativistic electrons in the Van Allen radiation belts

Nature volume 515, pages 531534 (27 November 2014) | Download Citation


Early observations1,2 indicated that the Earth’s Van Allen radiation belts could be separated into an inner zone dominated by high-energy protons and an outer zone dominated by high-energy electrons. Subsequent studies3,4 showed that electrons of moderate energy (less than about one megaelectronvolt) often populate both zones, with a deep ‘slot’ region largely devoid of particles between them. There is a region of dense cold plasma around the Earth known as the plasmasphere, the outer boundary of which is called the plasmapause. The two-belt radiation structure was explained as arising from strong electron interactions with plasmaspheric hiss just inside the plasmapause boundary5, with the inner edge of the outer radiation zone corresponding to the minimum plasmapause location6. Recent observations have revealed unexpected radiation belt morphology7,8, especially at ultrarelativistic kinetic energies9,10 (more than five megaelectronvolts). Here we analyse an extended data set that reveals an exceedingly sharp inner boundary for the ultrarelativistic electrons. Additional, concurrently measured data11 reveal that this barrier to inward electron radial transport does not arise because of a physical boundary within the Earth’s intrinsic magnetic field, and that inward radial diffusion is unlikely to be inhibited by scattering by electromagnetic transmitter wave fields. Rather, we suggest that exceptionally slow natural inward radial diffusion combined with weak, but persistent, wave–particle pitch angle scattering deep inside the Earth’s plasmasphere can combine to create an almost impenetrable barrier through which the most energetic Van Allen belt electrons cannot migrate.

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We thank the entire Van Allen Probes mission team for suggestions about this work. Data access was provided through the Johns Hopkins University/Applied Physics Lab Mission Operations Center and the Los Alamos National Laboratory Science Operations Center. This work was supported by JHU/APL contract 967399 under NASA’s prime contract NAS5-01072. All Van Allen Probes data used are publicly available at http://www.rbsp-ect.lanl.gov.

Author information


  1. Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80303, USA

    • D. N. Baker
    • , A. N. Jaynes
    • , V. C. Hoxie
    • , X. Li
    • , Q. Schiller
    • , L. Blum
    •  & D. M. Malaspina
  2. Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, California 90095, USA

    • R. M. Thorne
    • , W. Li
    •  & Q. Ma
  3. Massachusetts Institute of Technology, Haystack Observatory, Westford, Massachusetts 01886, USA

    • J. C. Foster
    •  & P. J. Erickson
  4. Aerospace Corporation Space Sciences Lab, Los Angeles, California 90009, USA

    • J. F. Fennell
  5. School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455, USA

    • J. R. Wygant
  6. NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA

    • S. G. Kanekal
  7. Department of Physics, University of Iowa, Iowa City, Iowa 52242, USA

    • W. Kurth
  8. Center for Solar-Terrestrial Research, New Jersey Institute of Technology, Newark, New Jersey 07102, USA

    • A. Gerrard
    •  & L. J. Lanzerotti


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D.N.B. developed the project, directed the data analysis and was primarily responsible for writing the paper. A.N.J., V.C.H. and S.G.K. analysed REPT data and produced related figures. R.M.T. provided theoretical guidance. J.C.F. and P.J.E. provided ground-based data for context. J.F.F. provided access to supplementary Van Allen Probes particle data. X.L., L.B. and Q.S. provided REPTile data. D.M.M. provided plasmapause location from EFW data. J.R.W. provided electric field data and W.K. provided EMFISIS data access. W.L. performed hiss data statistical analysis. Q.M. performed particle scattering and diffusion lifetime calculations. A.G. and L.J.L. provided ERM data from the Van Allen Probes mission.

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The authors declare no competing financial interests.

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Correspondence to D. N. Baker.

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