Letter | Published:

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

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

Abstract

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.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

References

  1. 1.

    & Particle acceleration at astrophysical shocks: A theory of cosmic ray origin. Phys. Rep. 154, 1–75 (1987).

  2. 2.

    et al. High-energy particle acceleration in the shell of a supernova remnant. Nature 432, 75–77 (2004).

  3. 3.

    , , , & Extremely fast acceleration of cosmic rays in a supernova remnant. Nature 449, 576–578 (2007).

  4. 4.

    & Quasi-perpendicular shock acceleration of ions to 200 MeV and electrons to 2 MeV observed by Voyager 2. Astrophys. J. 298, 676–683 (1985).

  5. 5.

    , , & Suprathermal electrons at Earth’s bow shock. J. Geophys. Res. 94, 10011–10025 (1989).

  6. 6.

    Voyager energetic particle observations at interplanetary shocks and upstream of planetary bow shocks: 1977–1990. Space Sci. Rev. 59, 167–201 (1992).

  7. 7.

    et al. Diffusive shock acceleration of electrons at an interplanetary shock observed on 21 Feb 1994. Astro. Space Sci. 264, 481–488 (1999).

  8. 8.

    et al. Whistler critical Mach number and electron acceleration at the bow shock: Geotail observation. Geophys. Res. Lett. 33, L24104 (2006).

  9. 9.

    Fundamentals of collisionless shocks for astrophysical application, 1. Non-relativistic shocks. Astron. Astrophys. Rev. 17, 409–535 (2009).

  10. 10.

    Supernova remnants at high energy. Annu. Rev. Astron. Astrophys. 46, 89–126 (2008).

  11. 11.

    Supernova remnants: The X-ray perspective. Astron. Astrophys. Rev. 20, 49–168 (2012).

  12. 12.

    in Collisionless Shocks in the Heliosphere: Reviews of Current Research (eds Tsurutani, B. T. & Stone, R. G.) 69–83 (Geophysical Monograph Series 35, American Geophysical Union, 1985).

  13. 13.

    in Collisionless Shocks in the Heliosphere: Reviews of Current Research (eds Tsurutani, B. T. & Stone, R. G.) 109–130 (Geophysical Monograph Series 35, American Geophysical Union, 1985).

  14. 14.

    Electron injection in collisionless shocks. Astrphys. J. 401, 73–80 (1992).

  15. 15.

    & A critical Mach number for electron injection in collisionless shocks. Phys. Rev. Lett. 104, 181102 (2010).

  16. 16.

    et al. Orientation, location, and velocity of Saturn’s bow shock: Initial results from the Cassini spacecraft. J. Geophys. Res. 111, A03201 (2006).

  17. 17.

    , , , & A new semiempirical model of Saturn’s bow shock based on propagated solar wind parameters. J. Geophys. Res. 116, A07202 (2011).

  18. 18.

    et al. Electron heating at Saturn’s bow shock. J. Geophys. Res. 116, A10107 (2011).

  19. 19.

    et al. The foreshock. Space Sci. Rev. 118, 41–94 (2005).

  20. 20.

    et al. Quasi-parallel shock structure and processes. Space Sci. Rev. 118, 205–222 (2005).

  21. 21.

    et al. Cassini plasma spectrometer investigation. Space Sci. Rev. 114, 1–112 (2004).

  22. 22.

    et al. Magnetosphere imaging instrument (MIMI) on the Cassini mission to Saturn/Titan. Space Sci. Rev. 114, 233–329 (2004).

  23. 23.

    et al. Analysis of a sequence of energetic ion and magnetic field events upstream from the Saturnian magnetosphere. Planet. Space Sci. 57, 1785–1794 (2009).

  24. 24.

    et al. The Cassini magnetic field investigation. Space Sci. Rev. 114, 331–383 (2004).

  25. 25.

    et al. The Cassini radio and plasma wave investigation. Space Sci. Rev. 114, 395–463 (2004).

Download references

Acknowledgements

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

Affiliations

  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

Authors

  1. Search for A. Masters in:

  2. Search for L. Stawarz in:

  3. Search for M. Fujimoto in:

  4. Search for S. J. Schwartz in:

  5. Search for N. Sergis in:

  6. Search for M. F. Thomsen in:

  7. Search for A. Retinò in:

  8. Search for H. Hasegawa in:

  9. Search for B. Zieger in:

  10. Search for G. R. Lewis in:

  11. Search for A. J. Coates in:

  12. Search for P. Canu in:

  13. Search for M. K. Dougherty in:

Contributions

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.

About this article

Publication history

Received

Accepted

Published

DOI

https://doi.org/10.1038/nphys2541

Further reading