ABSTRACT Highly energetic electrons are trapped in the magnetic field of Earth’s radiation belts. The physical mechanisms driving the dynamics of the Van Allen belts can be understood from

the electron’s energy spectrum, which is believed to be steeply falling with increasing energy. This view has been prevalent for the past 60 years since the energy spectra were first

measured. Here, we report the observation of a reversed energy spectrum with abundant high-energy and fewer low-energy electrons spanning from hundreds of keV to around two MeV in electron

energy in data collected with NASA’s Van Allen Probes. We find that this spectrum dominates inside the plasmasphere—a dense cold plasma region co-rotating with the Earth. Using

plasmas throughout the solar system and beyond. Access through your institution Buy or subscribe This is a preview of subscription content, access via your institution ACCESS OPTIONS Access

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support SIMILAR CONTENT BEING VIEWED BY OTHERS PARTICLE-SOUNDING OF THE SPATIAL STRUCTURE OF KINETIC ALFVÉN WAVES Article Open access 12 April 2023 DIRECT OBSERVATIONS OF ENERGY TRANSFER

FROM RESONANT ELECTRONS TO WHISTLER-MODE WAVES IN MAGNETOSHEATH OF EARTH Article Open access 28 October 2022 CHORUS WAVE POWER AT THE STRONG DIFFUSION LIMIT OVERCOMES ELECTRON LOSSES DUE TO

STRONG DIFFUSION Article Open access 27 February 2024 DATA AVAILABILITY The particle data analysed during the current study are available from the ECT Science Operations and Data Center

(http://www.rbsp-ect.lanl.gov). The field data are available from the EMFISIS Data Center (https://emfisis.physics.uiowa.edu/). Solar wind data and geomagnetic indices are available from

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on THEMIS measurements. _J. Geophys. Res. Space Phys._ 120, 10543–10556 (2015). Download references ACKNOWLEDGEMENTS This work was supported by RBSP-ECT funding through JHU/APL contract

967399 (under prime NASA contract NAS5-01072), by the NSFC grants 41674163, 41474141, 41574160 and 41204120, by the Chinese Thousand Youth Talents Program, and by the Hubei Province Natural

Science Excellent Youth Foundation (2016CFA044). We thank the Van Allen Probes REPT, MagEIS, HOPE and EMFISIS Science Teams for providing the particle and wave data. We also thank the NSSDC

OMNIWeb for the use of solar wind and geomagnetic index data. AUTHOR INFORMATION Author notes * These authors contributed equally: H. Zhao, B. Ni. AUTHORS AND AFFILIATIONS * Laboratory for

Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA H. Zhao, X. Li, D. N. Baker & Z. Xiang * School of Electronic Information, Wuhan University, Wuhan, China B. Ni, 

W. Zhang, Z. Xiang & X. Gu * Air Force Research Laboratory, Kirtland Air Force Base, Albuquerque, NM, USA W. R. Johnston * Department of Physics and Astronomy, University of Iowa, Iowa

City, IA, USA A. N. Jaynes * NASA Goddard Space Flight Center, Greenbelt, MD, USA S. G. Kanekal * Space Sciences Department, Aerospace Corporation, Los Angeles, CA, USA J. B. Blake & S.

G. Claudepierre * Space Sciences Laboratory, University of California, Berkeley, CA, USA M. A. Temerin * Los Alamos National Laboratory, Los Alamos, NM, USA H. O. Funsten & G. D. Reeves

* New Mexico Consortium, Los Alamos, NM, USA G. D. Reeves & A. J. Boyd Authors * H. Zhao View author publications You can also search for this author inPubMed Google Scholar * B. Ni 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 * D. N. Baker

View author publications You can also search for this author inPubMed Google Scholar * W. R. Johnston View author publications You can also search for this author inPubMed Google Scholar *

W. Zhang View author publications You can also search for this author inPubMed Google Scholar * Z. Xiang View author publications You can also search for this author inPubMed Google Scholar

* X. Gu 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 * S. G. Kanekal View author publications You can also search for this author inPubMed Google Scholar * J. B. Blake View author publications You can also search for this author

inPubMed Google Scholar * S. G. Claudepierre View author publications You can also search for this author inPubMed Google Scholar * M. A. Temerin View author publications You can also search

for this author inPubMed Google Scholar * H. O. Funsten View author publications You can also search for this author inPubMed Google Scholar * G. D. Reeves View author publications You can

also search for this author inPubMed Google Scholar * A. J. Boyd View author publications You can also search for this author inPubMed Google Scholar CONTRIBUTIONS H.Z. led the study,

performed the data analysis of the bump-on-tail energy spectrum and wrote the manuscript. B.N. initialized the concept of hiss wave scattering, led the simulations and their quantitative

comparisons with observations, and contributed to writing of the manuscript through reviews and edits. X.L. and D.N.B. supervised the study and contributed to writing of the manuscript

through reviews and edits. W.R.J. initialized the concept of bump-on-tail energy spectrum, helped with data analysis, and contributed to writing of the manuscript through reviews and edits.

W.Z. conducted the Fokker–Planck simulation runs to investigate the hiss-induced electron dynamics and produced the majority of Figs. 3 and 4. Z.X. provided the wave information to establish

the data-driven, time-dependent hiss wave model and helped produce Fig. 3. X.G. helped analyse the simulation results and compare them with observations. A.N.J., S.G.K., J.B.B., S.G.C.,

M.A.T., H.O.F., G.D.R. and A.J.B. contributed to writing of the manuscript through reviews and edits. CORRESPONDING AUTHORS Correspondence to H. Zhao or B. Ni. ETHICS DECLARATIONS COMPETING

INTERESTS The authors declare no competing interests. ADDITIONAL INFORMATION PUBLISHER’S NOTE: Springer Nature remains neutral with regard to jurisdictional claims in published maps and

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ARTICLE CITE THIS ARTICLE Zhao, H., Ni, B., Li, X. _et al._ Plasmaspheric hiss waves generate a reversed energy spectrum of radiation belt electrons. _Nat. Phys._ 15, 367–372 (2019).

https://doi.org/10.1038/s41567-018-0391-6 Download citation * Received: 17 May 2018 * Accepted: 30 November 2018 * Published: 21 January 2019 * Issue Date: April 2019 * DOI:

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