To investigate the universality of magnetic turbulence in space plasmas, we analyze seven time periods in the free solar wind under different plasma conditions. Three instruments on Cluster spacecraft operating in different frequency ranges give us the possibility to resolve spectra up to 300 Hz. We show that the spectra form a quasiuniversal spectrum following the Kolmogorov's law approximately k(-5/3) at MHD scales, a approximately k(-2.8) power law at ion scales, and an exponential approximately exp[-sqrt[k(rho)e]] at scales k(rho)e approximately [0.1,1], where rho(e) is the electron gyroradius. This is the first observation of an exponential magnetic spectrum in space plasmas that may indicate the onset of dissipation. We distinguish for the first time between the role of different spatial kinetic plasma scales and show that the electron Larmor radius plays the role of a dissipation scale in space plasma turbulence.

The solar-wind magnetohydrodynamic turbulence is observed to be mainly made of Alfvenic fluctuations propagating away from the sun. It is shown that such an asymmetric state is a general consequence of the evolution of developed magnetohydrodynamic turbulence, which, starting from an initial asymmetry between modes with cross helicity +1 and -1, tends, as a consequence of nonlinear interactions, towards a state where the only modes left are those initially prevailing (with either cross helicity +1 or -1).PACS numbers: 96.50.Dj, 96.60.Vg Theoretical investigations of strong hydromagnetic turbulence have always dealt so far with the isotropic case" and most often with the case where the average magnetic field is zero. '4 In the latter case, Kraichnan' has derived, using dimensional arguments, a 0 ' ' power law for the spectrum of the magnetic and kinetic energy densities of the fluctuations in the stationary state. The difference in the spectral index with respect to that of the Kolmogorov spectrum of isotropic hydrodynamic turbulence is due to the presence, in the smaller scales, of Alfvdn waves propagating in the magnetic field of the larger-scale eddies, thus impeding the energy transfer in this range of high wave numbers.Observations of incompressible magnetohydrodynamic (MHD) turbulence in the magnetized plasma of interplanetary space' ' indicate, however, ' that the existence of these Alfvd'n waves is not the only peculiar feature of MHD with respect to hydrodynamic turbulence.On the one hand, the spectral energy density of magnetic fluctuations E(k) defined by (5B')/4vp= f F(k) dk(1) (p being the plasma mass density) seems to follow a power law E(k)~k " with a spectral index v ranging from 1.2 to 2, for frequencies between 10 ' and 10 Hz. Although the scatter of the observed values of v precludes a definite identification with either a Kolmogorov or a Kraichnan spectrum, the observed power law is expected to result from a nonlinear energy cascade. 5v = + &B/(4 tt p) (2) the sign depending on the polarity of the average magnetic field and being such that only Alfvdnic fluctuations propagating away from the sun are observed. Notice that, in terms of the so-called cross helicity of hydromagnetic turbulence, " the observational result (2) implies that the MHD turbulence in the solar wind is either in a state characterized by the value +1 for the cross helicity, or in a -1 state.It is a simple matter to show that, if condition (2) is satisfied, there are no longer nonlinear interactions which is in apparent contrast with the presence of a spectrum. To see this, we write the equations for incompressible MHD fluctuations as" where 1 ( B ) (4) and C"=(B)/(4zp)~' is the Alfvenic speed in the average field (B). The above equations refer to Qn the other hand, in the same domain of wave vectors, and mainly in the trailing edges of fast solar-wind streams, one observes a striking correlation between the velocity 5v and magnetic fluctuations 6B which satisfy to a good degree the relation 144

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This paper introduces and describes the radio and plasma wave investigation on the STEREO Mission: STEREO/WAVES or S/WAVES. The S/WAVES instrument includes a suite of state-of-the-art experiments that provide comprehensive measurements of the three components of the fluctuating electric field from a fraction of a hertz up to 16 MHz, plus a single frequency channel near 30 MHz. The instrument has a direction finding or goniopolarimetry capability to perform 3D localization and tracking of radio emissions associated with streams of energetic electrons and shock waves associated with Coronal Mass Ejections (CMEs). The scientific objectives include: (i) remote observation and measurement of radio waves excited by energetic particles throughout the 3D heliosphere that are associated with the CMEs and with solar flare phenomena, and (ii) in-situ measurement of the properties of CMEs and interplanetary shocks, such as their electron density and temperature and the associated plasma waves near 1 Astronomical Unit (AU). Two companion papers provide details on specific aspects of the S/WAVES instrument, namely the electric antenna system (Bale et al., Space Sci. Rev., 2007) and the direction finding technique (Cecconi et al., Space Sci. Rev., 2007).

A comprehensive set of experimental observations of a high β (2.4), supercritical (Mf = 3.8), quasi‐perpendicular (ΘBn1 ∼ 76°) bow shock layer is presented, and its local geometry, spatial scales, and stationarity are assessed in a self‐consistent, Rankine‐Hugoniot‐constrained frame of reference. Included are spatial profiles of the ac and dc magnetic and electric fields, electron and proton fluid velocities, current densities, electron and proton number densities, temperatures, pressures, and partial densities of the reflected protons. The transformation of the apparent time scales to the actual spatial scales is performed with unprecedented accuracy. The observed layer profile is shown to be nearly phase standing and one dimensional in a Rankine‐Hugoniot frame, empirically determined by the magnetofluid parameters outside the layer proper. One or both of these properties appear to collapse at the time resolution of 1.5 s in the specific geometry considered in this study. Several pieces of evidence are used to show this stationarity: (1) the similarity of the average magnetic structures seen on the two ISEE spacecraft; (2) the close agreement between the electric currents directly determined from the plasma data and those inferred from the magnetic data assuming the layer is one dimensional and time stationary; (3) the close agreement of the empirically determined scale lengths of the most prominent substructures with those determined by numerical simulations and previous laboratory studies; and (4) the close agreement between the theoretical Rankine‐Hugoniot‐determined normal plasma pressure jump and that of the observed electron and proton fluids. The resolved cross‐field electrical currents (with empirical error estimates) are observed to peak within the main magnetic ramp at a level well below the first stabilization threshold for ion acoustic turbulence suggested for low β shocks by Galeev (1976); clear evidence is also provided for smaller parallel currents throughout the main ramp and overshoot, with a predominant sense as if the shock electric field has caused the lighter electrons to lead the ions along the local magnetic field direction. The width of the shock depends on what structures are used to define it. The upstream pedestal or “foot” is nearly two upstream ion skin depths wide, but the main magnetic ramp is only 1/5 the upstream ion skin depth and thus considerably smaller than “conventional wisdom” and most simulations. The ramp scale length is directly corroborated by the current densities determined from the plasma instruments.

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We present a study of magnetic field fluctuations, in a slow solar wind stream, close to ion scales, where an increase of the level of magnetic compressibility is observed. Here, the nature of these compressive fluctuations is found to be characterized by coherent structures. Although previous studies have shown that current sheets can be considered as the principal cause of intermittency at ion scales, here we show for the first time that, in the case of the slow solar wind, a large variety of coherent structures contributes to intermittency at proton scales, and current sheets are not the most common. Specifically, we find compressive (δb ≫ δb ⊥ ), linearly polarized structures in the form of magnetic holes, solitons and shock waves. Examples of Alfvénic structures (δb ⊥ > δb ) are identified as current sheets and vortex-like structures. Some of these vortices have δb ⊥ ≫ δb , as in the case of Alfvén vortices, but the majority of them are characterized by δb ⊥ δb . Thanks to multipoint measurements by Cluster spacecraft, for about 100 structures, we could determine the normal, the propagation velocity and the spatial scale along this normal. Independently of the nature of the structures, the normal is always perpendicular to the local magnetic field, meaning that k ⊥ ≫ k . The spatial scales of the studied structures are found to be between 2 and 8 times the proton gyroradius. Most of them are simply convected by the wind, but 25% propagate in the plasma frame. Possible interpretations of the observed structures and the connection with plasma heating are discussed.