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Measurement of electrons from albedo neutron decay and neutron density in near-Earth space


The Galaxy is filled with cosmic-ray particles, mostly protons with kinetic energies greater than hundreds of megaelectronvolts. Around Earth, trapped energetic protons, electrons and other particles circulate at altitudes from about 500 to 40,000 kilometres in the Van Allen radiation belts. Soon after these radiation belts were discovered six decades ago, it was recognized that the main source of inner-belt protons (with kinetic energies of tens to hundreds of megaelectronvolts) is cosmic-ray albedo neutron decay (CRAND)1. In this process, cosmic rays that reach the upper atmosphere interact with neutral atoms to produce albedo neutrons, which, being prone to β-decay, are a possible source of geomagnetically trapped protons and electrons. These protons would retain most of the kinetic energy of the neutrons, while the electrons would have lower energies, mostly less than one megaelectronvolt. The viability of CRAND as an electron source has, however, been uncertain, because measurements have shown that the electron intensity in the inner Van Allen belt can vary greatly, while the neutron-decay rate should be almost constant2,3. Here we report measurements of relativistic electrons near the inner edge of the inner radiation belt. We demonstrate that the main source of these electrons is indeed CRAND, and that this process also contributes to electrons in the inner belt elsewhere. Furthermore, measurement of the intensity of electrons generated by CRAND provides an experimental determination of the neutron density in near-Earth space—2 × 10−9 per cubic centimetre—confirming theoretical estimates4.

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Figure 1: Electron flux as a function of longitude in different L shells for the period 4–14 January 2013.
Figure 2: Dependence of an electron’s drift path on its altitude and L shell.
Figure 3: Electron flux as a function of longitude in different L shells for the period 7–10 October 2012.
Figure 4: Calculations showing the negligent effect of drift-shell splitting.


  1. 1

    Singer, S. F. “Radiation belt” and trapped cosmic-ray albedo. Phys. Rev. Lett. 1, 171 (1958)

    ADS  Article  Google Scholar 

  2. 2

    Kellogg, P. J. Van Allen radiation of solar origin. Nature 183, 1295–1297 (1959)

    ADS  Article  Google Scholar 

  3. 3

    Lenchek, A. M., Singer, S. F. & Wentworth, R. C. Geomagnetically trapped electrons from cosmic ray albedo neutrons. J. Geophys. Res. 66, 4027–4046 (1961)

    ADS  Article  Google Scholar 

  4. 4

    Hess, W. N., Canfield, E. H. & Lingenfelter, R. E. Cosmic-ray neutron demography. Geophys. Res. Lett. 66, 665–677 (1961)

    Article  Google Scholar 

  5. 5

    Li, X. et al. Colorado Student Space Weather Experiment: differential flux measurements of energetic particles in a highly inclined low Earth orbit. Geophys. Monogr. Ser. 199, 385–404 (2012)

    Google Scholar 

  6. 6

    Li, X. et al. First results from CSSWE CubeSat: characteristics of relativistic electrons in the near-Earth environment during the October 2012 magnetic storms. J. Geophys. Res. Space Phys. 118, 6489–6499 (2013)

    CAS  ADS  Article  Google Scholar 

  7. 7

    Li, X. et al. Upper limit on the inner radiation belt MeV electron intensity. J. Geophys. Res. Space Phys. 120, 1215–1228 (2015)

    CAS  ADS  Article  Google Scholar 

  8. 8

    Boscher, D., Bourdarie, S., Guild, T., O’Brein, P. & Heynderickx, D. International Radiation Belt Environment Modeling (IRBEM) library v.4.4, Toulouse, France (Panel on Radiation Belt Environment Modelling (PRBEM) Committee on Space Research (COSPAR), 2012)

  9. 9

    Selesnick, R. S. et al. Inward diffusion and loss of radiation belt protons. J. Geophys. Res. Space Phys. 121, 1969–1978 (2016)

    ADS  Article  Google Scholar 

  10. 10

    Roederer, J. G., Hilton, H. H. & Schulz, M. Drift shell splitting by internal geomagnetic multipoles. J. Geophys. Res. 78, 133–144 (1973)

    ADS  Article  Google Scholar 

  11. 11

    Selesnick, R. S. & Blake, J. B. Relativistic electron drift shell splitting. J. Geophys. Res. 107 (A9), 1265 (2002)

    Article  Google Scholar 

  12. 12

    Selesnick, R. S., Su, Y.-J. & Blake, J. B. Control of the innermost electron radiation belt by large-scale electric fields. J. Geophys. Res. Space Phys. 121, 8417–8427 (2016)

    ADS  Article  Google Scholar 

  13. 13

    Su, Y.-J., Selesnick, R. S. & Blake, J. B. Formation of the inner electron radiation belt by enhanced large-scale electric fields. J. Geophys. Res. Space Phys. 121, 8508–8522 (2016)

    ADS  Article  Google Scholar 

  14. 14

    Baker, D. N. et al. An extreme distortion of the Van Allen belt arising from the ‘Halloween’ solar storm in 2003. Nature 432, 878–881 (2004)

    CAS  ADS  Article  Google Scholar 

  15. 15

    Barker, A. B., Li, X. & Selesnick, R. S. Modeling the radiation belt electrons with radial diffusion driven by the solar wind. Space Weather 3, S10003 (2005)

    ADS  Article  Google Scholar 

  16. 16

    Li, X. et al. Modeling the deep penetration of outer belt electrons during the “Halloween” magnetic storm in 2003. Space Weather 7, S02004 (2009)

    ADS  Article  Google Scholar 

  17. 17

    Baker, D. N. et al. An impenetrable barrier to ultra-relativistic electrons in the Van Allen radiation belt. Nature 515, 531–534 (2014)

    CAS  ADS  Article  Google Scholar 

  18. 18

    Selesnick, R. S. Atmospheric scattering and decay of inner radiation belt electrons. J. Geophys. Res. 117, A08218 (2012)

    ADS  Article  Google Scholar 

  19. 19

    Selesnick, R. S. Stochastic simulation of inner radiation belt electron decay by atmospheric scattering. J. Geophys. Res. Space Phys. 121, 1249–1262 (2016)

    ADS  Article  Google Scholar 

  20. 20

    Sauvaud, J. A. et al. High energy electron detection onboard DEMETER: the IDP spectrometer, description and first results on the inner belt. Planet. Space Sci. 54, 502–511 (2006)

    ADS  Article  Google Scholar 

  21. 21

    Selesnick, R. S. High-energy radiation belt electrons from CRAND. J. Geophys. Res. Space Phys. 120, 2912–2917 (2015)

    ADS  Article  Google Scholar 

  22. 22

    Schiller, Q., Mahendrakumar, A. & Li, X. REPTile: a miniaturized detector for a CubeSat mission to measure relativistic particles in near-Earth space. In 24th Annual AIAA/USU Conference on Small Satellites SSC10-VIII-1 (2010)

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This work was supported in part by the National Science Foundation (NSF) CubeSat Program, NSF grant AGS 1443749, and NASA/Radiation Belt Storm Probes (RBSP)-Energetic particle, Composition and Thermal plasma (ECT) funding through Johns Hopkins University (JHU)/Applied Physics Laboratory (APL) contract 967399 under prime NASA contract NAS5-01072.

Author information




X.L. developed the project, directed the data analysis and was primarily responsible for writing the paper. R.S. was involved with the project from the beginning, gave advice on data analysis, and helped to revise the paper. Q.S. calibrated REPTile’s response and produced related data and figures. K.Z. and H.Z. performed data analysis and produced related figures. D.N.B. gave advice on data analysis and revision of the paper. M.A.T. gave advice on data analysis and helped to revise the paper.

Corresponding author

Correspondence to Xinlin Li.

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The authors declare no competing financial interests.

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Reviewer Information Nature thanks M. Hudson and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Figure 1 Confirmation of our findings from other satellite measurements.

Electron flux measured by the DEMTER satellite20, which has an altitude of 710 km and inclination of 98.3°, plotted versus geographic longitude for different energies (E) and different L bins, and averaged over every 10° for the time period 20–30 April 2010. The magnitude of flux of quasi-trapped electrons at 0.5 MeV is the same as that measured by REPTile.

Extended Data Figure 2 Flux conversion on the basis of the β-decay spectrum of albedo neutrons and the detector’s energy response.

The ambient electron flux (J; solid line) is normalized and calculated from equation (1). The dashed line, representing the known β-decay spectrum (φ(E)), is normalized to a maximum value of 1 (but the value of φ = 1.2 used in the text is normalized with the area under the curve that is equal to 1) and calculated using equation (4) of ref. 21). The response of REPTile channel 1 (Ch1) to normally incident electrons is shown by the dotted line. The y axis shows J, the normalized β-decay spectrum (maximum normalized to 1), and the channel 1 efficiency (maximum is 1).

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Li, X., Selesnick, R., Schiller, Q. et al. Measurement of electrons from albedo neutron decay and neutron density in near-Earth space. Nature 552, 382–385 (2017).

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