HR: 1340h
AN: SH53A-1247    [Abstracts]
TI: Microturbulence in the foot of a supercritical shock:evidence of electron cyclotron instability
AU: * Muschietti, L
EM: laurent@ssl.berkeley.edu
AF: Space Sciences Laboratory UC Berkeley, 7 Gauss Way, Berkeley, CA 94720-7450 United States
AU: Lembege, B
EM: lembege@cetp.ipsl.fr
AF: CETP-CNRS-UVSQ, 10--12 Av. de l'Europe, Velizy, F-78140 France
AB: We have performed one-dimensional full particle simulations of perpendicular shocks and found that an electron cyclotron microinstability can develop in the foot of supercritical shocks. For such shocks, a certain percentage of incoming ions are reflected and are responsible for the cyclic self-reformation of the shock front in low-β plasmas [1,2]. One surprising result is that this instability arises even when the supercritical shock has a relatively low (but still supercritical) Mach number. The instability is periodically excited by the beam of reflected ions interacting with the electrons during each self-reformation cycle. It exhibits a rapid growth, and propagates along the shock normal towards upstream. As its phase velocity is close to the beam velocity, it has a noticeable impact on both the populations of reflected ions and electrons, but does not interact with the incoming ions. This instability has a frequency comparable to the electron cyclotron and a wavelength shorter than the electron inertia length. It is mainly electrostatic and basically results from the coupling of electron Bernstein waves with an ion beam mode carried by the reflected ions. Dispersion properties encountered in the foot are analysed and are found in good agreement with results obtained self-consistently from the present simulations. A theoretical model is developed in order to estimate the instability wavelength versus the main plasma parameters. From this model, we analyze the effects of varying simulation parameters, as the fake ion-to-electron mass ratio and the reduced electron plasma-to-gyro frequency ratio used in the simulations converge to more realistic values. In particular, it is shown that raising the ion-to-electron mass ratio alone shortens the wavelength, while raising the other ratio alone elongates the wavelength. Thus, an increase in both ratios towards more realistic values compensate each other, which makes the electron cyclotron instability an interesting candidate for the electrostatic microturbulence in the shock front. [1] Hada et al., J. Geophys. Res., 108 (A6), 2003.
[5pt] [2] Scholer et al., J. Geophys. Res., 108 (A1), 2003.
DE: 2154 Planetary bow shocks
DE: 4455 Nonlinear waves, shock waves, solitons (0689, 2487, 3280, 3285, 4275, 6934, 7851, DE: 7851 Shock waves (4455)
DE: 7863 Turbulence (4490)
DE: 7867 Wave/particle interactions (2483, 6984)
SC: SPA-Solar and Heliospheric Physics [SH]
MN: Fall Meeting 2005