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Electron acceleration to relativistic energies at a strong quasi-parallel shock wave

Nature Physics volume 9, pages 164167 (2013) | Download Citation


Electrons can be accelerated to ultrarelativistic energies at strong (high Mach number) collisionless shock waves that form when stellar debris rapidly expands after a supernova1,2,3. Collisionless shock waves also form in the flow of particles from the Sun (the solar wind), and extensive spacecraft observations have established that electron acceleration at these shocks is effectively absent whenever the upstream magnetic field is roughly parallel to the shock-surface normal (quasi-parallel conditions)4,5,6,7,8. However, it is unclear whether this magnetic dependence of electron acceleration also applies to the far stronger shocks around young supernova remnants, where local magnetic conditions are poorly understood. Here we present Cassini spacecraft observations of an unusually strong solar system shock wave (Saturn’s bow shock) where significant local electron acceleration has been confirmed under quasi-parallel magnetic conditions for the first time, contradicting the established magnetic dependence of electron acceleration at solar system shocks4,5,6,7,8. Furthermore, the acceleration led to electrons at relativistic energies (about megaelectronvolt), comparable to the highest energies ever attributed to shock acceleration in the solar wind4. These observations suggest that at high Mach numbers, such as those of young supernova remnant shocks, quasi-parallel shocks become considerably more effective electron accelerators.

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A.M. acknowledges the support of the JAXA International Top Young Fellowship Program, and P. Gandhi for useful discussions. We thank Cassini instrument Principal Investigators D. A. Gurnett, S. M. Krimigis and D. T. Young. This work was supported by UK STFC through rolling grants to MSSL/UCL and Imperial College London.

Author information


  1. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan

    • A. Masters
    • , L. Stawarz
    • , M. Fujimoto
    •  & H. Hasegawa
  2. Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan

    • M. Fujimoto
  3. Space and Atmospheric Physics Group, The Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2AZ, UK

    • S. J. Schwartz
    •  & M. K. Dougherty
  4. Office of Space Research and Technology, Academy of Athens, Soranou Efesiou 4, 11527 Athens, Greece

    • N. Sergis
  5. Space Science and Applications, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

    • M. F. Thomsen
  6. Laboratoire de Physique des Plasmas, Centre National de la Recherche Scientifique, Observatoire de Saint-Maur, 4 avenue de Neptune, Saint-Maur-Des-Fossés 94107, France

    • A. Retinò
    •  & P. Canu
  7. Center for Space Physics, Boston University, 725 Commonwealth Avenue, Boston, Massachusetts 02215, USA

    • B. Zieger
  8. Mullard Space Science Laboratory, Department of Space and Climate Physics, University College London, Holmbury St. Mary, Dorking RH5 6NT, UK

    • G. R. Lewis
    •  & A. J. Coates
  9. The Centre for Planetary Sciences at UCL/Birkbeck, Gower Street, London WC1E 6BT, UK

    • G. R. Lewis
    •  & A. J. Coates


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A.M. identified the event, analysed the combined data set, proposed the interpretation and wrote the paper. L.S., M.F., S.J.S., H.H. and B.Z. discussed the interpretation. N.S., M.F.T., A.R. and G.R.L. each analysed, and checked the interpretation of, one data set. A.J.C., P.C. and M.K.D. oversaw the data analysis and interpretation. All authors reviewed the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to A. Masters.

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