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ABSTRACT: Electron scale solar wind turbulence has attracted great interest in recent
years. Clear evidences have been given from the Cluster data that turbulence is
not fully dissipated near the proton scale but continues cascading down to the
electron scales. However, the scaling of the energy spectra as well as the
nature of the plasma modes involved at those small scales are still not fully
determined. Here we survey 10 years of the Cluster search-coil magnetometer
(SCM) waveforms measured in the solar wind and perform a statistical study of
the magnetic energy spectra in the frequency range [$1, 180$]Hz. We show that a
large fraction of the spectra exhibit clear breakpoints near the electon
gyroscale $\rho_e$, followed by steeper power-law like spectra. We show that
the scaling below the electron breakpoint cannot be determined unambiguously
due to instrumental limitations that will be discussed in detail. We compare
our results to recent ones reported in other studies and discuss their
implication on the physical mechanisms and the theoretical modeling of energy
dissipation in the SW.
03/2013;
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ABSTRACT: Recent observations of the solar wind have pointed out the existence of a cascade of magnetic energy from the scale of the proton Larmor radius ρ_{p} down to the electron Larmor radius ρ_{e} scale. In this Letter we study the spatial properties of magnetic field fluctuations in the solar wind and find that at small scales the magnetic field does not resemble a sea of homogeneous fluctuations, but rather a two-dimensional plane containing thin current sheets and discontinuities with spatial sizes ranging from l≳ρ_{p} down to ρ_{e} and below. These isolated structures may be manifestations of intermittency that localize sites of turbulent dissipation. Studying the relationship between turbulent dissipation, reconnection, and intermittency is crucial for understanding the dynamics of laboratory and astrophysical plasmas.
Physical Review Letters 11/2012; 109(19):191101. · 7.37 Impact Factor
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Space Science Reviews 04/2012; 118(1):95-152. · 3.61 Impact Factor
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ABSTRACT: Magnetic turbulence in the solar wind has been studied for many years. Most of the observational work have been -5/3 focused on the large (MHD) scales, i.e. the so-called inertial range with a Kolmogorov scaling k<sup>-5/3</sup> [e.g.,1,2]. The inertial range is widely believed to form by strong nonlinear interaction of Alfvén waves. However, the anisotropy of the turbulence (in wavenumber space) is still hotly debated [e.g., 3]. From single spacecraft data, inferring the wavenumber spectra from the temporal measured ones onboard the satellite can be achieved only by using the Taylor frozen-in assumption ω<sub>sat</sub>~k.V<sub>sw</sub>. This means that all the phase speeds of the waves need to be smaller than the solar wind speed V<sub>sw</sub>. While this assumption is generally valid at MHD scales (because the Alfvén speed V<sub>A</sub> <;<;V<sub>sw</sub>), it breaks down at the sub-ion (and electron) scales where whistler modes may exit. Moreover, even when the Taylor assumption is justified it can yield only one component of the wavenumber spectra: along the flow V<sub>sw</sub> [4-6]. The two other directions perpendicular to V<sub>sw</sub> are thus missing unless additional assumptions, such as isotropy, are used. Therefore multispacraft data and appropriate space-correlations methods are necessary in order to fully determine the 3D wavenumber spectra of space turbulence. This can be achieved by applying the k-filtering technique on the four Cluster spacecraft data.
General Assembly and Scientific Symposium, 2011 XXXth URSI; 09/2011
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ABSTRACT: We show the first three dimensional (3D) dispersion relations and k spectra of magnetic turbulence in the solar wind at subproton scales. We used the Cluster data with short separations and applied the k-filtering technique to the frequency range where the transition to subproton scales occurs. We show that the cascade is carried by highly oblique kinetic Alfvén waves with ω(plas) ≤ 0.1ω(ci) down to k(⊥) ρ(i)∼2. Each k spectrum in the direction perpendicular to B0 shows two scaling ranges separated by a breakpoint (in the interval [0.4,1]k(⊥)ρ(i): a Kolmogorov scaling k(⊥)⁻¹ⁱ⁷ followed by a steeper scaling ∼k(⊥)⁻⁴ⁱ⁵. We conjecture that the turbulence undergoes a transition range, where part of the energy is dissipated into proton heating via Landau damping and the remaining energy cascades down to electron scales where electron Landau damping may predominate.
Physical Review Letters 09/2010; 105(13):131101. · 7.37 Impact Factor
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ABSTRACT: The anisotropic nature of solar wind magnetic turbulence fluctuations is
investigated scale-by-scale using high cadence in-situ magnetic field
measurements from the Cluster and ACE spacecraft missions. The data span five
decades in scales from the inertial range to the electron Larmor radius. In
contrast to the inertial range, there is a successive increase towards isotropy
between parallel and transverse power at scales below the ion Larmor radius,
with isotropy being achieved at the electron Larmor radius. In the context of
wave-mediated theories of turbulence, we show that this enhancement in magnetic
fluctuations parallel to the local mean background field is qualitatively
consistent with the magnetic compressibility signature of kinetic Alfven wave
solutions of the linearized Vlasov equation. More generally, we discuss how
these results may arise naturally due to the prominent role of the Hall term at
sub-ion Larmor scales. Furthermore, computing higher-order statistics, we show
that the full statistical signature of the fluctuations at scales below the ion
Larmor radius is that of a single isotropic globally scale-invariant process
distinct from the anisotropic statistics of the inertial range.
08/2010;
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ABSTRACT: Using four-point measurements of the Cluster spacecraft, the energy distribution was determined for magnetic field fluctuations in the solar wind directly in the three-dimensional wave-vector domain in the range |k|<or=1.5x10{-3} rad/km. The energy distribution exhibits anisotropic features characterized by a prominently extended structure perpendicular to the mean field preferring the ecliptic north direction and also by a moderately extended structure parallel to the mean field. From the three-dimensional energy distribution wave vector anisotropy is estimated with respect to directions parallel and perpendicular to the mean magnetic field, and the result suggests the dominance of quasi-two-dimensional turbulence toward smaller spatial scales.
Physical Review Letters 04/2010; 104(17):171101. · 7.37 Impact Factor
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ABSTRACT: The Kelvin-Helmholtz instability (KHI) has been considered as one of the
most dominant mechanisms by which the shocked solar wind enters the
almost stagnant magnetosphere at the low-latitude magnetopause. The KHI
can grow nonlinearly along the flank of the magnetopause to form
large-scale rolled-up vortices. These rolled-up Kelvin-Helmholtz
vortices (KHV) have thus far been detected only under northward
interplanetary magnetic field (IMF) conditions. In this paper, we
present first in-situ observation of rolled-up KHV during southward IMF
conditions, using data from a Cluster crossing of the dawn-side flank of
the magnetotail. The observation of a mixture of rolled-up and not
rolled-up vortices that show inconsistent variations in the scale size,
the magnetic perturbation, and the boundary normal direction indicates
that KHV under southward IMF might have more temporal or intermittent
nature, which might explain the preferential in-situ detection of KHV
under northward IMF conditions. As a consequence of the KHV nonlinear
growth, plasma transport and mixing are observed within or at the edge
of KHV where a variety of magnetic topologies are found, possibly due to
either diffusive transport via a turbulent decay of rolled-up KHV, or
anti-parallel and/or guide-field reconnection
AGU Fall Meeting Abstracts. 11/2009; -1:1530.
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ABSTRACT: A higher-order multiscale analysis of the dissipation range of collisionless plasma turbulence is presented using in situ high-frequency magnetic field measurements from the Cluster spacecraft in a stationary interval of fast ambient solar wind. The observations, spanning five decades in temporal scales, show a crossover from multifractal intermittent turbulence in the inertial range to non-Gaussian monoscaling in the dissipation range. This presents a strong observational constraint on theories of dissipation mechanisms in turbulent collisionless plasmas.
Physical Review Letters 08/2009; 103(7):075006. · 7.37 Impact Factor
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ABSTRACT: We report the first direct determination of the dissipation range of magnetofluid turbulence in the solar wind at the electron scales. Combining high resolution magnetic and electric field data of the Cluster spacecraft, we computed the spectrum of turbulence and found two distinct breakpoints in the magnetic spectrum at 0.4 and 35 Hz, which correspond, respectively, to the Doppler-shifted proton and electron gyroscales, f(rho p) and f(rho e). Below f(rho p), the spectrum follows a Kolmogorov scaling f (-1.62), typical of spectra observed at 1 AU. Above f (rho p), a second inertial range is formed with a scaling f;{-2.3} down to f (rho e). Above f (rho e), the spectrum has a steeper power law approximately f (-4.1) down to the noise level of the instrument. We interpret this as the dissipation range and show a remarkable agreement with theoretical predictions of a quasi-two-dimensional cascade into Kinetic Alfvén Waves (KAW).
Physical Review Letters 07/2009; 102(23):231102. · 7.37 Impact Factor
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ABSTRACT: Most of observational work on solar wind (SW) turbulence has been
devoted to large-scale/MHD scales where the Kolmogorov scaling k-5/3 is
frequently observed. Turbulence at frequencies above the proton
gyrofrequency (fci~0.1Hz) has not been thoroughly investigated and
remains far less well understood. Above fci the spectrum steepens to
~f-2.5 and a debate exists as to whether the turbulence has become
dominated by dispersive kinetic Alfvén waves and is dissipative,
or has evolved into a new dispersive turbulent cascade dominated by
whistler waves. Here we present recent results on the nature of this
small-scale turbulence (up to 100 Hz) using the high resolution STAFF-SC
data. These studies are made using two complementary methods: the
k-filtering and the surrogate data methods. The k-filtering is a
multipoint measurement technique that allows one to identify the nature
of the turbulence and to calculate 3D k-spectra from the omega-ones
(Sahraoui et al, PRL, 2006). To investigate small scale SW turbulence,
one needs to take advantage of the times when the separation of the four
Cluster spacecraft was small. Given separations of 200km, and a typical
speed of the solar wind of 600km/s, one expects to be able to study
frequencies down to 3Hz. The surrogate data technique (Sahraoui, PRE,
2008) completes the previous studies that use only the power spectra
(where the phases of the fluctuations are ignored) by making extensive
use of the Fourier phases of the turbulent signals to investigate
coherent structures and intermittency.
03/2009; 11:10179.
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ABSTRACT: The anisotropic nature of solar wind magnetic fl uctuations is investigated scale-by-scale using high cadence in-situ magnetic fi eld ACE, and Cluster FGM and STAFF observations spanning fi ve decades in scales from the inertial to dissipation ranges of plasma turbulence. We fi nd an abrupt transition at ion kinetic scales to a single isotropic stochastic process as characterized by the single functional form of the probability density functions (PDFs) of fl uctuations that characterizes the dissipation range on all observable scales. In contrast to the inertial range, this is accompanied by a successive scale-invariant reduction in the ratio between parallel and transverse power. We suggest that this refl ects the phase space nature of the cascade which operates in a scale-invariant isotropic manner in the (kinetic) dissipation range – distinct from the anisotropic phenomenology in the (magnetohydrodynamic) inertial range. Alternatively, if we assume that non-linear effects are weak in the dissipation range and use the results of the linear dispersion theory of waves; then our measurements of fl uctuation anisotropy can provide some insight into the nature of these waves. In particular, using these measurements to form a measure for the scale-by-scale magnetic compressibility, we can distinguish between the competing hypotheses of highly oblique kinetic Alfven waves versus Whistler waves dominating the energy transfer in the dissipation range.
Phys. Rev. Lett. Phys. Rev. Lett. JGR Phys. Rev. Lett. Phys. Rev. Lett. Phys. Rev. Lett. 01/2009; 1033(105):231102-12105.
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F Sahraoui
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ABSTRACT: Intermittency is usually identified in turbulent flows as non-Gaussian tails of the probability density functions (PDFs) of the turbulent field derivatives. Here we investigate the role of phase coherence among the Fourier modes in creating intermittency in magnetized space plasmas using the technique of surrogate data. We apply the technique to two examples: (i) synthetic data and (ii) magnetic field fluctuations recorded in the terrestrial magnetosheath by the Cluster spacecraft. We use a set of four series of data, one observed and three surrogate, and their PDFs and moments (q < or = 4) as discriminating statistics. We show that the technique allows for detecting coherent structures and estimating their scales. We show furthermore that the phases, but not the amplitudes, create the non-Gaussian tails of the PDFs. We show also that the surrogate data used cannot account for asymmetries of the PDFs of the observed data. This enables us to confirm a scenario of turbulent cascade of mirror structures proposed in previous publications, by showing the existence of an approximately constant energy flux in the inertial range.
Physical Review E 08/2008; 78(2 Pt 2):026402. · 2.26 Impact Factor
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ABSTRACT: Here we report the first three-dimensional spatial spectrum of the low frequency magnetic turbulence obtained from the four Cluster spacecraft in the terrestrial magnetosheath close to the magnetopause. We show that the turbulence is compressible and dominated by mirror structures, its energy is injected at a large scale kp approximately 0.3 (l approximately 2000 km) via a mirror instability well predicted by linear theory, and cascades nonlinearly and unexpectedly up to kp approximately 3.5 (l approximately 150 km), revealing a new power law in the inertial range not predicted by any turbulence theory, and its strong anisotropy is controlled by the static magnetic field and the magnetopause normal.
Physical Review Letters 03/2006; 96(7):075002. · 7.37 Impact Factor
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ABSTRACT: The k-filtering technique is a method to characterize stationary fluctuations in space plasmas in terms of the wave energy distribution in the frequency and wave vector space. It has the ability to distinguish between wave modes of the same frequency in the spacecraft frame of reference, but with different wave vectors. This method is based on simultaneous multi-point measurements of the wave field components, where a filter bank is used to enhance the spatial resolution. We have for the first time combined electric field data from the Electric Field and Wave (EFW) instrument and magnetic field data from the Spatio-Temporal Analysis of Field Fluctuation (STAFF) instrument on the four Cluster spacecraft in order to determine the wave energy distribution. The k-filtering technique has previously been performed with only the magnetic field measurements. The reason to include the electric field measurements is that it is important to include as much data as possible in order to get the best possible estimation of the wave energy distribution. Another reason is that it also enables comparisons between the electric and the magnetic part of the wave energy distribution to make it possible to differentiate the observed waves according to their polarization. The k-filtering method has been extended in order to allow for two measured components of the electric field, and also for the possibility that the electric field measurements from one or more satellites cannot be used for k-filtering. The technique has been applied on satellite data from the magnetosheath and the foreshock, and from these examples it is clear that k-filtering using both electric and magnetic field measurements is a good tool for characterizing the waves that are observed in space.
Journal of Geophysical Research (Space Physics). 11/2005; 110:11224.
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ABSTRACT: The different levels of description of fluid media [e.g., magnetohydrodynamics (MHD), Hall-magnetohydrodynamics, bi-fluid,…] are commonly known under the form of Newtonian systems of equations. Nevertheless, this form proves to be ill-suited to derive a fully analytical weak turbulence theory of these media, due to the well-known complexity of the calculations implied. For such studies, therefore, a more appropriate mathematical frame needs to be found and this is shown to be the Hamiltonian formalism, even though it can often appear difficult to handle. The goal of this paper is to look for Hamiltonian formulations for the different levels of the fluid description of a plasma using the variational principle. Starting from the bi-fluid system, it is shown that such a formulation can be obtained by combining the Lagrangians already used for describing: (i) the motion of a charged particle in an electromagnetic field; (ii) the evolution of an electromagnetic field in presence of sources; (iii) the motion of a neutral fluid (Clebsch variables). The equivalence of the obtained description in terms of the generalized-Clebsch variables to the familiar Newtonian formulation is discussed. It is shown that each solution of the Hamiltonian system is also a solution for the Newtonian one, but that the converse is not true. The origin and the implication of this restriction are discussed. Reducing the Hamiltonian formulation obtained for the bi-fluid system to lower orders of the fluid approximations is then shown to be mandatory when one tries to obtain analytical results for linear waves and nonlinear wave–wave couplings. It is shown that this goal can be reached in two steps. The first one leads to a “reduced bi-fluid” system, which is identical to the bi-fluid one when the displacement current is neglected but the electron inertia is still working. The number of linear modes then goes down from six to three. The second step, leading to the Hall-MHD system, consists in neglecting the electron mass. It is demonstrated that the only four generalized Clebsch variables are sufficient to describe the full Hall-MHD dynamics. Some future applications of such a powerful formalism are outlined. © 2003 American Institute of Physics.
Physics of Plasmas 04/2003; 10(5):1325-1337. · 2.15 Impact Factor
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L. Rezeau, F. Sahraoui,
E. D'Humières,
G. Belmont,
T. Chust,
N. Cornilleau-Wehrlin,
L. Mellul,
O. Alexandrova,
E. Lucek,
P Robert,
P. Décréau,
P. Canu,
I. Dandouras
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ABSTRACT: We present the study of one of the first magnetopause crossings observed by the four Cluster spacecraft simultaneously, on 10 December 2000. Although the delays between the crossings are very short, the features of the boundary appear quite different as seen by the different spacecraft, strongly suggesting the presence of a local curvature of the magnetopause at that time. The small-scale fluctuations observed by the STAFF search-coil experiment are placed in relation to this context. A preliminary investigation of their behaviour on the boundary and in the neighbourhood magnetosheath is performed in comparison with the theoretical model of Belmont and Rezeau (2001), which describes the interaction of waves with the boundary.
01/2001;
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ABSTRACT: Cluster spacecraft crossed the high-altitude (11 Re) cusp region during
March 2002. The CIS instrument identified both field aligned particles
and ions that display high levels of isotropisation (stagnant plasma).
Multi-instrument Cluster data are used in order to highlight this
phenomenon. During large part of the studied event, the STAFF and EFW
instruments have detected strong electromagnetic wave emissions at low
frequencies. Focus on an ion injection period highlights the role of the
waves in the isotropisation of the distribution function. The
characteristics of the waves are studied using the k-filtering method.
Preliminary result shows that the low frequency (at about 0.5Hz) part of
the electromagnetic spectrum is alfvènic.
35:2594.
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ABSTRACT: The four point measurements available from the Cluster mission enable spatiotemporal effects in data sets to be resolved. One application of these multipoint measurements is the determination of the wave vectors and hence the identification of wave modes that exist within the plasma. Prior to multi-satellite missions, wave identification techniques were based upon the interpretation of observational data using theoretically defined relations. However, such techniques are limited by the quality of the data and the type of plasma model employed. With multipoint measurements, wave modes can be identified and their wave directions determined purely from the available observations. This paper takes two such methods, a phase differencing technique and k-filtering and compares their results. It is shown that both methods can resolve the k vector for the dominant mirror mode present in the data. The phase differencing method shows that the nature of the wave environment is constantly changing and as such both methods result in an average picture of the wave environment in the period analysed. The k-filtering method is able to identify other modes that are present.
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ABSTRACT: On 23 March 2002, the four Cluster spacecraft crossed in close configuration (~100 km separation) the high-altitude (10 R<sub>E</sub> ) cusp region. During a large part of the crossing, the STAFF and EFW instruments have detected strong electromagnetic wave activity at low frequencies, especially when intense field-aligned proton fluxes were detected by the CIS/HIA instrument. In all likelihood, such fluxes correspond to newly-reconnected field lines. A focus on one of these ion injection periods highlights the interaction between waves and protons. The wave activity has been investigated using the k-filtering technique. Experimental dispersion relations have been built in the plasma frame for the two most energetic wave modes. Results show that kinetic Alfvén waves dominate the electromagnetic wave spectrum up to 1 Hz (in the spacecraft frame). Above 0.8 Hz, intense Bernstein waves are also observed. The close simultaneity observed between the wave and particle events is discussed as an evidence for local wave generation. A mechanism based on current instabilities is consistent with the observations of the kinetic Alfvén waves. A weak ion heating along the recently-opened field lines is also suggested from the examination of the ion distribution functions. During an injection event, a large plasma convection motion, indicative of a reconnection site location, is shown to be consistent with the velocity perturbation induced by the large-scale Alfvén wave simultaneously detected.
