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).

Measurements of the total energy, cross helicity, and magnetic helicity of the solar wind at 1, 2.8, and 5 AU are presented. These quantities are the three rugged invariants of three‐dimensional ideal incompressible MHD turbulence theory. The theoretical technique for measuring the magnetic helicity from the matrix of two‐point correlations is shown. The length scales characterizing the magnetic helicity are found to be equal to or greater than those which characterize the magnetic energy. The magnetic helicity typically lies at scales larger than the magnetic correlation length, consistent with the expectations of the inverse cascade and selective decay hypotheses of three‐dimensional MHD turbulence. At smaller scales, the magnetic helicity oscillates in sign. Our measurements of the cross helicity are not fully consistent with the usual interpretation in terms of outward propagating Alfvénic functuations. Especially during the interval at 5 AU the cross helicity is found to oscillate in sign indicating fluctuations propagating both outward and inward.

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.

Solar wind fluctuations are commonly regarded as a superposition of MHD waves, primarily in the Alfvén mode. These MHD fluctuations are frequently assumed to possess “slab” or isotropic symmetry, particularly in the development of models of the propagation of cosmic rays throughout the heliosphere. There are, however, several long‐standing problems with either of these choices. One problem is that the mean free path for pitch angle scattering of cosmic rays in the heliosphere is apparently longer than can be accounted for by using either assumption about the statistical symmetry of the fluctuations. Another problem is the prediction of WKB theory that the direction of minimum variance should tend to lie along the radial direction rather than along the mean magnetic field as is observed. Motivated by laboratory plasma experiments, a series of two‐dimensional MHD simulations, recent theoretical work, and extensive analyses of solar wind data, we suggest that there is a third possible viewpoint with potentially important implications for solar wind studies. From this perspective we suggest that solar wind fluctuations contain a subpopulation that have wave vectors nearly transverse to both the mean magnetic field and the fluctuations about the mean. For this quasi‐two‐dimensional component the direction of minimum variance lies along the mean magnetic field, density fluctuations are small and anticorrelated with |B|, the total pressure at small scales is nearly constant, and pitch angle scattering by resonant wave‐particle interactions is suppressed.

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NASA’s Solar Probe Plus (SPP) mission will make the first in situ measurements of the solar corona and the birthplace of the solar wind. The FIELDS instrument suite on SPP will make direct measurements of electric and magnetic fields, the properties of in situ plasma waves, electron density and temperature profiles, and interplanetary radio emissions, amongst other things. Here, we describe the scientific objectives targeted by the SPP/FIELDS instrument, the instrument design itself, and the instrument concept of operations and planned data products.

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Abstract. The Cluster mission,