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.
Subscribe to Journal
Get full journal access for 1 year
only $3.90 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Tax calculation will be finalised during checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
Van Allen, J. A., Ludwig, G. H., Ray, E. C. & McIlwain, C. E. Observation of high intensity radiation by satellites 1958 alpha and gamma. Jet Propuls. 28, 588–592 (1958)
Van Allen, J. A. & Frank, L. A. Radiation around the Earth to a radial distance of 107,400 km. Nature 183, 430–434 (1959)
Johnson, M. H. & Kierein, J. Combined Release and Radiation Effects Satellite (CRRES): spacecraft and mission. J. Spacecr. Rockets 29, 556–563 (1992)
Blake, J. B., Baker, D. N., Turner, N., Ogilvie, K. W. & Lepping, R. P. Correlation of changes in the outer-zone relativistic electron population with upstream solar wind and magnetic field measurements. Geophys. Res. Lett. 24, 927–929 (1997)
Lyons, L. R. & Thorne, R. M. Equilibrium structure of radiation belt electrons. J. Geophys. Res. 78, 2142–2149 (1973)
Li, X., Baker, D. N., O’Brien, P., Xie, L. & Zong, Q. G. Correlation between the inner edge of outer radiation belt electrons and the innermost plasmapause location. Geophys. Res. Lett. 33, L14107 (2006)
Baker, D. N. et al. A long-lived relativistic electron storage ring embedded in Earth’s outer Van Allen belt. Science 340, 186–190 (2013)
Thorne, R. M. et al. Rapid local acceleration of relativistic radiation belt electrons by magnetospheric chorus. Nature 504, 411–414 (2013)
Baker, D. N. et al. The Relativistic Electron-Proton Telescope (REPT) instrument on board the Radiation Belt Storm Probes (RBSP) spacecraft: characterization of Earth’s radiation belt high-energy particle populations. Space Sci. Rev. 179, 337–381 (2013)
Spence, H. E. et al. Science goals and overview of the Radiation Belt Storm Probes (RBSP) Energetic Particle, Composition, and Thermal Plasma (ECT) suite on NASA’s Van Allen Probes mission. Space Sci. Rev. 179, 311–336 (2013)
Mauk, B. H. et al. Science objective and rationale for the Radiation Belt Storm Probes mission. Space Sci. Rev. 179, 3–27 (2013)
Blake, J. B., Kolasinski, W. A., Fillius, R. W. & Mullen, E. G. Injection of electrons and protons with energies of tens of MeV into L < 3 on March 24, 1991. Geophys. Res. Lett. 19, 821–824 (1992)
Baker, D. N. et al. An extreme distortion of the Van Allen belt arising from the ‘Hallowe’en’ solar storm in 2003. Nature 432, 878–881 (2004)
Baker, D. N., Kanekal, S. G., Horne, R. B., Meredith, N. P. & Glauert, S. A. Low-altitude measurements of 2–6 MeV electron trapping lifetimes at 1.5 ≤ L ≤ 2.5. Geophys. Res. Lett. 34, L20110 (2007)
Zhao, H. & Li, X. Inward shift of outer radiation belt electrons as a function of Dst index and the influence of the solar wind on electron injections into the slot region. J. Geophys. Res. Space Phys. 118, 756–764 (2013)
Kletzing, C. A. et al. The Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) on RBSP. Space Sci. Rev. 179, 127–181 (2013)
Roederer, J. G. Dynamics of Geomagnetically Trapped Radiation (Springer, 1970)
Vampola, A. L. & Kuck, G. A. Induced precipitation of inner zone electrons, 1. Observations. J. Geophys. Res. 83, 2543–2551 (1978)
Koons, H. C., Edgar, B. C. & Vampola, A. L. Precipitation of inner zone electrons by Whistler mode waves from the VLF transmitters UMS and NWC. J. Geophys. Res. 86, 640–648 (1981)
Abel, R. W. & Thorne, R. M. Electron scattering loss in Earth’s inner magnetosphere: 1. Dominant physical processes. J. Geophys. Res. 103, 2385–2396 (1998)
Li, X. et al. Simulation of the prompt energization and transport of radiation particles during the March 23, 1991 SSC. Geophys. Res. Lett. 20, 2423–2426 (1993)
Baker, D. N. et al. Gradual diffusion and punctuated phase space density enhancements of highly relativistic electrons: Van Allen Probes observations. Geophys. Res. Lett. 41, 1351–1358 (2014)
Ni, B., Bortnik, J., Thorne, R. M., Ma, Q. & Chen, L. Resonant scattering and resultant pitch angle evolution of relativistic electrons by plasmaspheric hiss. J. Geophys. Res. 118, 7740–7751 (2013)
Thorne, R. M. et al. Evolution and slow decay of an unusual narrow ring of relativistic electrons near L ∼ 3.2 following the September 2012 magnetic storm. Geophys. Res. Lett. 40, 3507 (2013)
Eastlund, B. J., Miller, B. & Michel, F. C. Emission from closed and filled magnetospheric shells and its application to the Crab pulsar. Astrophys. J. 483, 857–867 (1997)
NASA Goddard Space Flight Center. Coordinated Data Analysis Web, http://cdaweb.gsfc.nasa.gov/istp_public/ (11 September 2014)
Brautigam, D. H. & Albert, J. Radial diffusion analysis of outer radiation belt electrons during the October 9, 1990 magnetic storm. J. Geophys. Res. 105, 291–309 (2000)
Goldsten, J. O. et al. The Engineering Radiation Monitor for the Radiation Belt Storm Probes Mission. Space Sci. Rev. 179, 485–502 (2013)
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)
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.
The authors declare no competing financial interests.
Extended data figures and tables
Extended Data Figure 1 Induced charge monitor current density measured by Van Allen Probes spacecraft A.
The first seven months of the mission are shown as a function of dipole L shell. Time is measured from 1 January 2013. The charge plate of the Environmental Radiation Monitor28 (ERM) from which the data shown here were acquired consists of a 10 cm2 plate under 1 mm-thick aluminium, and is thus able to detect penetrating electrons of more than 0.7 MeV and protons of more than 15 MeV. Note that no enhanced charging was observed below L ≈ 3 for solar active periods throughout October 2012 (about day −80) and April 2013 (about day +80). Similar results (not shown) were observed in charge monitor 2, which was under 3.8 mm of aluminium and was thus able to detect penetrating electrons of more than 2 MeV and protons of more than 30 MeV. Source data
Extended Data Figure 2 Data from the Colorado Student Space Weather Experiment CubeSat mission in low-Earth orbit.
The REPT integrated little experiment (REPTile) >3.8 MeV electron data are portrayed in a latitude–longitude Mercator projection format showing that the electron inner edge of the outer zone is well separated from the SAA (which is dominated for this energy range in REPTile by inner-zone protons)29.Data are from ref. 26.
Extended Data Figure 3 Pitch angle data exhibiting the behaviour of high-energy-electron angular distributions.
This figure shows illustrative data26 in the storage ring region and at the inner edge of the outer zone for an entire Van Allen Probe A orbital pass for February 2013 from 21:30 ut on 15 February to about 5:00 ut on 16 February. a, Colour-coded directional fluxes for 2.0 MeV electrons. b, Similar data for 2.8 MeV electrons. c, Similar data for 4.5 MeV electrons. d, Similar data for 5.6 MeV electrons. e, Similar data for 7.2 MeV electrons.
Data measured by REPT-A for 15–18 February 2013 for different spacecraft orbit numbers are colour-coded according to the inset in c. a, Distributions seen right at the inner edge of the trapping boundary at L = 2.8. b, Distributions taken in the higher-flux regions at L = 3.0. c, Distributions taken even further out in the trapping region at L = 3.2. Source data
Extended Data Figure 5 A colour-coded geographic representation of ultrarelativistic electron fluxes.
The orbital tracks of Van Allen Probe B for the REPT-B sensor fluxes from 1 September to 28 September 2013 are projected onto the geographical equatorial plane. As the spacecraft precesses in its elliptical orbit around the Earth, it forms a ‘Spirograph’ pattern in the geographically fixed, Earth-centred coordinate system. The resulting orbital pattern shows the relatively stable (during this 4-week period) band of 7.2 MeV electrons from a radius of about 2.8 Earth radii (RE) out to about 3.5RE. Inside 2.8RE there is an almost complete absence of electrons, resulting in the slot region. Note also that there is hardly any discernible population of electrons at these energies in the inner zone (L ≤ 2) during this period. The superimposed circle at 2.8RE shows how sharp and distinctive the inner boundary is for ultrarelativistic electrons and how generally symmetric this boundary is all around the Earth. Source data
About this article
Cite this article
Baker, D., Jaynes, A., Hoxie, V. et al. An impenetrable barrier to ultrarelativistic electrons in the Van Allen radiation belts. Nature 515, 531–534 (2014). https://doi.org/10.1038/nature13956
The Astrophysical Journal (2021)
A review of the SCOSTEP’s 5-year scientific program VarSITI—Variability of the Sun and Its Terrestrial Impact
Progress in Earth and Planetary Science (2021)
Journal of Geophysical Research: Space Physics (2021)
Journal of Geophysical Research: Space Physics (2021)
The Astrophysical Journal (2020)