Skip to main content

Volume 61 Supplement 5

Special Issue: Flare-Substorm/Space Weather Topics

Non-thermal electrons at the Earth’s bow shock: A ‘gradual’ event

Abstract

The Earth’s bow shock is known to produce non-thermal electrons which are generally observed as a ‘spike’ in their flux profile. Here, in this paper, we present an analysis of electron and whistler wave properties for a quasi-perpendicular shock crossing that is supercritical, but subcritical to the so-called whistler critical Mach number, M wcrit , above which whistler waves cannot propagate upstream. We have found that the amplitudes of whistler waves increased exponentially as a function of time prior to the shock encounter, while the suprathermal (>2 keV) electron flux similarly increased with time, although with differing e-folding time scales. Comparison of the electron energy spectrum measured within the ramp with predictions from diffusive shock acceleration theory was poor, but the variation of pitch angle distribution showed scattering of non-thermal electrons in the upstream region. While not finding a specific mechanism to account for the electron diffusion, we suggest that the whistlers seen probably account for the differences observed between this ‘gradual’ event and the ‘spike’ events seen at shocks with no upstream whistlers.

References

  • Anderson, K. A., Energetic electrons of terrestrial origin behind the bow shock and upstream in the solar wind, J. Geophys. Res., 74, 95, 1969.

    Article  Google Scholar 

  • Anderson, K. A. et al., Thin sheets of energetic electrons upstream from the Earth’s bow shock, Geophys. Res. Lett., 6, 401, 1979.

    Article  Google Scholar 

  • Blandford, R. D. and J. P. Ostriker, Particle acceleration by astrophysical shocks, Astrophys. J., 221, L29–L32, 1978.

    Article  Google Scholar 

  • Fairfield, D. H., Whistler waves observed upstream from collisionless shocks, J. Geophys. Res., 79, 1368–1378, 1974.

    Article  Google Scholar 

  • Fan, C. Y. et al., Evidence for >30keV electrons accelerated in the shock transition region beyond the Earth’s magnetospheric boundary, Phys. Rev. Lett., 13, 149, 1964.

    Article  Google Scholar 

  • Frank, L. A. and J. A. Van Allen, Measurements of energetic electrons in the vicinity of the sunward magnetospheric boundary with Explorer 14, J. Geophys. Res., 69, 4923, 1964.

    Article  Google Scholar 

  • Giacalone, J., Particle acceleration at shocks moving through an irregular magnetic field, Astrophys. J., 624, 765, 2005.

    Article  Google Scholar 

  • Gosling et al., Suprathermal electrons at Earth’s bow shock, J. Geophys. Res., 94, 10,011–10,025, 1989.

    Article  Google Scholar 

  • Horbury et al., Four spacecraft measurements of the quasiperpendicular terrestrial bow shock: Orientation and motion, J. Geophys. Res., 107, 273, 2002.

    Google Scholar 

  • Jokipii, J. R. and J. Giacalone, Adiabatic compression acceleration of fast charged particles, Astrophys. J., 660, 336, 2007.

    Article  Google Scholar 

  • Kasaba, Y. et al., Statistical studies of plasma waves and backstreaming electrons in the terrestrial electron foreshock observed by Geotail, J. Geophys. Res., 105, 79–103, 2000.

    Article  Google Scholar 

  • Kokubun et al., The GEOTAIL magnetic-field experiment, J. Geomag. Geoelectr., 46, 7–21, 1994.

    Article  Google Scholar 

  • Matsui, H. et al., Long-duration whistler waves in the magnetosheath: Wave characteristics and the possible source region, J. Geophys. Res., 102, 17,583–17,593, 1997.

    Article  Google Scholar 

  • Means, J. D., Use of three-dimensional covariance matrix in analyzing the polarization properties of plane waves, J. Geophys. Res., 77, 5551, 1972.

    Article  Google Scholar 

  • Mukai, T. et al., The Low Energy Particle (LEP) experiment onboard the GEOTAIL satellite, J. Geomag. Geoelectr., 46, 669–692, 1994.

    Article  Google Scholar 

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

    Article  Google Scholar 

  • Orlowski, D. S. et al., On the source of upstream whistlers in the Venus foreshock, in COSPAR Colloquia, Plasma environments of nonmagnetic Planets, vol. 4, edited by T. I. Gombosi, 217–227, Pergamon Press, New York, 1994.

    Google Scholar 

  • Orlowski, D. S. et al., Damping and spectral formation of upstream whistlers, J. Geophys. Res., 100, 17,117–17,128, 1995.

    Article  Google Scholar 

  • Paschmann, G. and P. W. Daly, Analysis methods for multi-spacecraft data, ISSI Scientific Report, SR-001, pp. 536, 1998.

    Google Scholar 

  • Peredo, M. et al., Three-dimensional position and shape of the bow shock and their variation with Alfvenic, sonic and magnetosonic Mach numbers and interplanetary magnetic field orientation, J. Geophys. Res., 100, 7907–7916, 1995.

    Article  Google Scholar 

  • Sentman, D. D. et al., The oblique whistler instability in the earths fore-shock, J. Geophys. Res., 88, 2048–2056, 1983.

    Article  Google Scholar 

  • Sonnerup, B. U. Ö. and L. J. Cahill Jr., Magnetopause structure and attitude from Explorer 12 observations, J. Geophys. Res., 72, 171–183, 1967.

    Article  Google Scholar 

  • Tokar, R. L. et al., Whistler mode turbulence generated by electron beams in the bow shock, J. Geophys. Res., 89, 105, 1984.

    Article  Google Scholar 

  • Vandas, M., Acceleration of electrons by a nearly perpendicular Earth’s bow shock: A comparison between observation and theory, Bull. Astron. Inst. Czech, 40, 175–188, 1989.

    Google Scholar 

Download references

Author information

Authors and Affiliations

Authors

Corresponding author

Correspondence to M. Oka.

Rights and permissions

Open Access  This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.

The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/.

Reprints and Permissions

About this article

Cite this article

Oka, M., Terasawa, T., Fujimoto, M. et al. Non-thermal electrons at the Earth’s bow shock: A ‘gradual’ event. Earth Planet Sp 61, 603–606 (2009). https://doi.org/10.1186/BF03352932

Download citation

  • Received:

  • Revised:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1186/BF03352932

Key words

  • Particle acceleration
  • scattering
  • bow shock
  • whistlers