An impenetrable barrier to ultrarelativistic electrons in the van allen radiation belts

ABSTRACT 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

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FAQs * Contact customer support SIMILAR CONTENT BEING VIEWED BY OTHERS LOCAL HEATING OF RADIATION BELT ELECTRONS TO ULTRA-RELATIVISTIC ENERGIES Article Open access 10 September 2020

LIGHTNING-INDUCED RELATIVISTIC ELECTRON PRECIPITATION FROM THE INNER RADIATION BELT Article Open access 08 October 2024 EARTH’S AMBIPOLAR ELECTROSTATIC FIELD AND ITS ROLE IN ION ESCAPE TO

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CAS  Google Scholar  Download references ACKNOWLEDGEMENTS 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 AUTHORS AND AFFILIATIONS * Laboratory for

Atmospheric and Space Physics, University of Colorado, Boulder, 80303, Colorado, USA D. N. Baker, A. N. Jaynes, V. C. Hoxie, X. Li, Q. Schiller, L. Blum & D. M. Malaspina * Department of

Atmospheric and Oceanic Sciences, University of California, Los Angeles, 90095, California, USA R. M. Thorne, W. Li & Q. Ma * Massachusetts Institute of Technology, Haystack

Observatory, Westford, 01886, Massachusetts, USA J. C. Foster & P. J. Erickson * Aerospace Corporation Space Sciences Lab, Los Angeles, 90009, California, USA J. F. Fennell * School of

Physics and Astronomy, University of Minnesota, Minneapolis, 55455, Minnesota, USA J. R. Wygant * NASA Goddard Space Flight Center, Greenbelt, 20771, Maryland, USA S. G. Kanekal * Department

of Physics, University of Iowa, Iowa City, 52242, Iowa, USA W. Kurth * Center for Solar-Terrestrial Research, New Jersey Institute of Technology, Newark, 07102, New Jersey, USA A. Gerrard 

& L. J. Lanzerotti Authors * D. N. Baker View author publications You can also search for this author inPubMed Google Scholar * A. N. Jaynes View author publications You can also search

for this author inPubMed Google Scholar * V. C. Hoxie View author publications You can also search for this author inPubMed Google Scholar * R. M. Thorne View author publications You can

also search for this author inPubMed Google Scholar * J. C. Foster View author publications You can also search for this author inPubMed Google Scholar * X. Li View author publications You

can also search for this author inPubMed Google Scholar * J. F. Fennell View author publications You can also search for this author inPubMed Google Scholar * J. R. Wygant View author

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Schiller View author publications You can also search for this author inPubMed Google Scholar * L. Blum View author publications You can also search for this author inPubMed Google Scholar *

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Google Scholar * L. J. Lanzerotti View author publications You can also search for this author inPubMed Google Scholar CONTRIBUTIONS 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. CORRESPONDING AUTHOR Correspondence to D. N. Baker. ETHICS DECLARATIONS COMPETING

INTERESTS 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. EXTENDED DATA FIGURE 4 DIFFERENTIAL DIRECTIONAL FLUX VALUES VERSUS PITCH ANGLE VALUES. 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 (_R_E)

out to about 3.5_R_E. Inside 2.8_R_E 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 2.8_R_E 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 POWERPOINT SLIDES POWERPOINT SLIDE FOR FIG. 1 POWERPOINT SLIDE FOR FIG. 2 POWERPOINT SLIDE FOR FIG. 3 POWERPOINT

SLIDE FOR FIG. 4 SOURCE DATA SOURCE DATA TO FIG. 1 SOURCE DATA TO FIG. 2 SOURCE DATA TO EXTENDED DATA FIG. 3 SOURCE DATA TO EXTENDED DATA FIG. 4 SOURCE DATA TO EXTENDED DATA FIG. 5 RIGHTS

AND PERMISSIONS Reprints and permissions 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 Download citation * Received: 18 June 2014 * Accepted: 10 October 2014 * Published: 26 November 2014

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