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.
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.
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.
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.
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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
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