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Ann. Geophys., 24, 3507-3521, 2006
www.ann-geophys.net/24/3507/2006/
© European Geosciences Union 2006
Cluster observations in the magnetosheath – Part 1: Anisotropies of the wave vector distribution of the turbulence at electron scales
A. Mangeney1, C. Lacombe1, M. Maksimovic1, A. A. Samsonov2, N. Cornilleau-Wehrlin3, C. C. Harvey4, J.-M. Bosqued4, and P. Trávníček5
1LESIA/CNRS, Observatoire de Paris, Meudon, France
2Institute of Physics, St. Petersburg State University, St. Petersburg, Russia
3Centre d'étude des Environnements Terrestre et Planétaire/UVSQ, Vélizy, France
4Centre d'Etude Spatiale des Rayonnements/CNRS, Toulouse, France
5Institute of Atmospheric Physics, Prague, Czech Republic
Abstract. We analyse the power spectral density δB2 and δE2 of the
magnetic and electric fluctuations measured by Cluster 1 (Rumba) in the magnetosheath
during 23 h, on four different days. The frequency range of the STAFF
Spectral Analyser (f=8 Hz to 4 kHz) extends from about the lower hybrid
frequency, i.e. the electromagnetic (e.m.) range, up to about 10 times
the proton plasma frequency, i.e. the electrostatic (e.s.) range. In the
e.m. range, we do not consider the whistler waves, which are not always
observed, but rather the underlying, more permanent fluctuations. In this e.m. range,
δB2 (at 10 Hz) increases strongly while the local angle
ΘBV between the magnetic field B and the flow velocity
V increases from 0° to 90°. This behaviour, also
observed in the solar wind at lower frequencies, is due to the Doppler effect.
It can be modelled if we assume that, for the scales ranging from
kc/ωpe≃0.3 to 30
(c/ωpe is the electron inertial length), the intensity of the e.m.
fluctuations for a wave number k (i) varies like k−ν with
ν>≃3, (ii) peaks for wave vectors k perpendicular to
B like |sinθkB|µ with µ>≃100. The shape of
the observed variations of δB2 with f and with ΘBV
implies that the permanent fluctuations, at these scales, statistically do not
obey the dispersion relation for fast/whistler waves or for kinetic Alfvén waves:
the fluctuations have a vanishing frequency in the plasma frame, i.e. their
phase velocity is negligible with respect to V (Taylor hypothesis). The
electrostatic waves around 1 kHz behave differently: δE2 is minimum
for ΘBV>≃90°. This can be modelled, still with
the Doppler effect, if we assume that, for the scales ranging from
k λDe>≃0.1 to 1 (λDe is the Debye length),
the intensity of the e.s. fluctuations (i) varies like k−ν with
ν>≃4, (ii) peaks for k parallel to B like
|cosθkB|µ with µ>≃100. These e.s. fluctuations may
have a vanishing frequency in the plasma frame,
or may be ion acoustic waves. Our observations
imply that the e.m. frequencies observed in the magnetosheath result from the
Doppler shift of a spatial turbulence frozen in the plasma, and that the
intensity of the turbulent k spectrum is strongly anisotropic, for
both e.m. and e.s. fluctuations. We conclude that the turbulence has
strongly anisotropic k distributions, on scales ranging
from kc/ωpe≃0.3 (50 km) to kλDe≃1 (30 m),
i.e. at electron scales, smaller than the Cluster separation.
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