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Earth Planets Space, Vol. 61 (No. 5), pp. 603-606, 2009

LETTER

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

M. Oka1, T. Terasawa2, M. Fujimoto3, H. Matsui4, Y. Kasaba5, Y. Saito3, H. Kojima6, H. Matsumoto7, and T. Mukai8

1Kwasan Observatory, Kyoto University, Yamashina-ku, Kyoto, Japan
2Tokyo Institute of Technology, Meguro-ku, Tokyo, Japan
3Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Kanagawa, Japan
4University of New Hampshire, Durham, New Hampshire, USA
5Tohoku University, Sendai, Japan
6Research Institute for Sustainable Humanosphere, Kyoto University, Uji, Kyoto, Japan
7Kyoto University, Sakyo-ku, Kyoto, Japan
8Japan Aerospace Exploration Agency, Chofu, Tokyo, Japan

(Received February 18, 2008; Revised April 6, 2008; Accepted May 19, 2008; Online published May 29, 2009)

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, Mwcrit, 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.
Key words: Particle acceleration, scattering, bow shock, whistlers.


Corresponding author E-mail: mitsuo.oka@uah.edu


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