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AGU: Geophysical Research Letters

 

Keywords

  • bow shock
  • electron acceleration
  • whistler waves

Index Terms

  • Interplanetary Physics: Energetic particles
  • Interplanetary Physics: Plasma waves and turbulence
  • Space Plasma Physics: Particle acceleration
  • Space Plasma Physics: Shock waves
  • Space Plasma Physics: Wave/particle interactions

Abstract

GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L24104, 4 PP., 2006
doi:10.1029/2006GL028156

Whistler critical Mach number and electron acceleration at the bow shock: Geotail observation

M. Oka

Kwasan Observatory, Kyoto University, Kyoto, Japan

Institute of Space and Astronomical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan

Department of Earth and Planetary Science, University of Tokyo, Tokyo, Japan

T. Terasawa

Tokyo Institute of Technology, Tokyo, Japan

Y. Seki

Institute of Space and Astronomical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan

M. Fujimoto

Institute of Space and Astronomical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan

Y. Kasaba

Institute of Space and Astronomical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan

H. Kojima

Radio Atmospheric Science Center, Kyoto University, Kyoto, Japan

I. Shinohara

Institute of Space and Astronomical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan

H. Matsui

Space Science Center, University of New Hampshire, Durham, New Hampshire, USA

H. Matsumoto

Research Institute for Sustainable Humanosphere, Kyoto University, Kyoto, Japan

Y. Saito

Institute of Space and Astronomical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan

T. Mukai

Institute of Space and Astronomical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan

The ‘whistler critical Mach number’, M crit w , is one of the dimensionless parameters that characterizes collisionless shocks. Originally, it was introduced to indicate the critical point above which whistler waves do not propagate upstream. Indeed our analysis of Geotail data at the Earth's bow shock shows intense whistler waves in the sub-critical regime, M A < M crit w , but not in the super-critical regime. In this paper, we further report that M crit w seems to regulate the electron acceleration efficiency at the shocks. At the shock transition layer it is found that the spectral index Γ of electron energy spectra defined by f(E) ∝ E −Γ is distributed between 3.5 and 5.0 in the sub-critical regime, while the hardest energy spectra with Γ = 3–3.5 are detected in the super-critical regime. We discuss a possible relationship between M crit w and the electron acceleration.

Received 13 September 2006; accepted 22 November 2006; published 28 December 2006.

Citation: Oka, M., et al. (2006), Whistler critical Mach number and electron acceleration at the bow shock: Geotail observation, Geophys. Res. Lett., 33, L24104, doi:10.1029/2006GL028156.

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