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Ann. Geophys., 24, 3523-3531, 2006
www.ann-geophys.net/24/3523/2006/
© European Geosciences Union 2006
Cluster observations in the magnetosheath – Part 2: Intensity of the turbulence at electron scales
C. Lacombe1, A. A. Samsonov2, A. Mangeney1, M. Maksimovic1, 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. The Cluster STAFF Spectral Analyser measures the magnetic and electric power
spectral densities (PSD) δB2 and δE2 in the magnetosheath
between 8 Hz and 4 kHz, i.e. between about the lower hybrid frequency
and 10 times the proton plasma frequency. We study about 23 h
of data on four different days. We do not consider the whistler waves and the
electrostatic pulses (which are not always observed)
but the underlying permanent fluctuations. Paper 1 (Mangeney et al.,
2006) shows why the permanent PSD at a given frequency f depends
strongly on the angle ΘBV between the magnetic field B and the
flow velocity V: this is observed for the electromagnetic (e.m.)
fluctuations, δB2 and δEem2, below the electron cyclotron
frequency fce, and for the electrostatic (e.s.) fluctuations
δEes2 at and above fce. This dependence is due to the Doppler
shift of fluctuations which have a highly anisotropic distribution of the
intensity of the wave vector k spectrum, and have a power law intensity
∝k−ν with ν≃3 to 4. In the present paper, we look for
parameters, other
than ΘBV, which control the intensity of the fluctuations. At
f≃10 Hz, δB2 and δE2em increase when the solar
wind dynamic pressure PDYNSW increases. When PDYNSW increases,
the magnetosheath PDYNMS∝N V2 also increases, so that
the local Doppler shift (k.V) increases for a given k. If
V increases, a given frequency f will be reached by fluctuations with a
smaller k, which are more intense: the variations of δB2 (10 Hz)
with PDYNSW are only due to the Doppler shift in the spacecraft frame.
We show that the e.m. spectrum in the plasma frame has an invariant shape
I1D∝Aem (kc/ωpe)−ν related to the electron
inertial length c/ωpe: the intensity Aem does not depend
on PDYN, nor on the electron to proton temperature ratio
Te/Tp, nor on the upstream bow shock angle θBN.
Then, we show results of 3-D MHD numerical
simulations of the magnetosheath plasma, which map the regions
where the angle ΘBV is ≃90°. The e.m. fluctuations
are more intense in these magnetosheath regions, in the spacecraft frame where
they are observed in the "whistler" range; and the e.s. fluctuations are less
intense in these same regions, in the spacecraft frame where they are observed
in the "ion acoustic" range. We conclude that the intensity of the permanent
fluctuations in the e.m. range only depends on the Doppler shift, so
that from day to day and from place to place in the magnetosheath, the
k spectrum in the plasma frame has an invariant shape and a constant
intensity. This is observed on scales ranging from
kc/ωpe≃0.3 (50 km) to kc/ωpe≃30 (500 m),
i.e. at electron scales smaller than the Cluster separation.
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