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Ann. Geophys., 25, 1157-1173, 2007
www.ann-geophys.net/25/1157/2007/
© European Geosciences Union 2007
Proton temperature anisotropy in the magnetosheath: comparison of 3-D MHD modelling with Cluster data
A. A. Samsonov1, O. Alexandrova2, C. Lacombe2, M. Maksimovic2, and S. P. Gary3
1Institute of Physics, St. Petersburg State University, St. Petersburg, Russia
2LESIA/CNRS, Observatoire de Paris, Meudon, France
3Los Alamos National Laboratory, Los Alamos, New Mexico, USA
Abstract. We study four intervals of Cluster data, lasting from five
to eight hours, in the flanks of the magnetosheath. In a first
part, we make numerical simulations of these magnetosheath
crossings, using a three-dimensional double-adiabatic MHD model of
the magnetosheath and assuming that the proton temperature
anisotropy is bounded by the kinetic thresholds of the Alfvén
proton cyclotron instability and of the mirror instability. The
conditions at the upstream boundary of the numerical domain are
given by the solar wind parameters observed by ACE. We assume that
the magnetopause is a fixed and impenetrable boundary, i.e.
without magnetic reconnection. The global agreement between the
observations and the simulations confirms the validity of the
model in the magnetosheath flanks. We discuss the consequences of
different models of the magnetopause on some simulation results.
In a second part, we compare the observed proton temperature
anisotropy and the kinetic anisotropy thresholds of the two
above-mentioned instabilities which are local functions of the
proton β. In the intervals with a low proton β, the
observed temperature anisotropy agrees well with the kinetic
threshold of the proton-cyclotron instability; in the intervals
with a higher β, the observed anisotropy is close to both
the proton-cyclotron and the mirror thresholds. This confirms that
the observed proton anisotropy is indeed bounded by the
instability thresholds. We then analyse the magnetic field power
spectra in a frequency range 0.003–10 Hz during four 18-min
intervals for different values of β. If β<1,
transverse (i.e. Alfvénic) fluctuations are dominant at every
frequency. For β≥1, a mixture of compressive (i.e.
mirror) and transverse waves is usually observed. For a case with
β≃10, there is no frequency where compressive waves
are dominant. The values of β and of the proton temperature
anisotropy are thus important but not the only parameters which
determine the dominant mode, compressive or transverse, at the
proton scales in the magnetosheath.
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