SH41C-01
Anisotropy and Dissipation in Space Plasma Turbulence: Cluster Observations in the Solar Wind and Earth's Magnetosheath
The solar wind is the most studied laboratory of space plasma turbulence. Here, at MHD scales, the Kolmogorov's power law f-5/3 is observed for Alfvénic fluctuations, which dominate the turbulent spectrum. Above the spectral break in the vicinity of the ion cyclotron frequency fci, the spectrum becomes steeper, ~ f-3, and the level of compressible fluctuations raises. This range is usually called dissipation range of solar wind turbulence. Using Cluster Search Coil magnetometer data, we show that the statistical properties of the fluctuations above the spectral break (at f>fci) are inherent to an inertial and not to the dissipative range. The analysis of Cluster Spectrum Analyser data (spectra from 8~Hz up to 4~kHz) is performed in order to possibly establish the starting scale of the dissipation range. The Earth's magnetosheath is another example of magnetic turbulence in space plasmas. Here, the Kolmogorov power law at f<fci is observed only in the flanks, however, the small scale inertial range, with the same spectrum as in the solar wind ~ f-3, is generally observed. We study the anisotropy of wave vector distributions of these small scale fluctuations, using a statistical method, based on the dependence of the observed magnetic energy at a fixed frequency on the Doppler shift for different wavenumbers. Thus we show that this small scale fluctuations are dominated by 2D turbulence with wave vectors mainly perpendicular to a mean field B, k⊥≫ k|. Such anisotropy is observed up to electron scales and it is independent on the value of plasma β, which varies in our analysis from less than 1 to about 10.
SH41C-02 INVITED
What did Cluster discover in the solar wind?
The Cluster mission consists of four grouped spacecraft which are observing the Earth's magnetosphere and the near-Earth solar wind. Their unique configuration provides full three dimensional spatial resolution in space plasma, and Cluster can be used as a powerful tool to study physical processes in solar wind turbulence. One of the aims of turbulence studies is to estimate physical quantities like energy directly in three dimensional wave number domain. Although measurements at four points are not sufficient to perform the Fourier transform from spatial coordinates into wave vectors, a projection method called the wave telescope can be used as its equivalent alternative in order to determine experimentally dispersion relations, energy distributions, energy spectra, and bispectra in the wave number domain. We review these analysis methods on an elementary level as well as applications to and results from Cluster observations, which provides us with a new picture of solar wind turbulence.
SH41C-03
Quasilinear Dissipation and Focusing of Oblique Ion Cyclotron Waves by Coronal Hole Protons
Perpendicular proton heating in the low-beta solar coronal hole is observed by the UVCS/SOHO instrument and is likely responsible for the acceleration of the fast solar wind. Resonant cyclotron dissipation of turbulently generated Alfvén ion cyclotron waves is often invoked to provide this heating, but the necessary turbulent cascade of energy to resonant parallel scales is not understood. Additionally, Isenberg (2004) showed that dissipation of strictly parallel-propagating ion cyclotron waves is limited due to the presence of a marginally stable state of the proton distribution. In this talk, we demonstrate that proton absorption of oblique waves has no such limitation since it scatters the protons away from this marginal stability. We also find that the subsequently heated proton distribution will trend back to the stable state by emitting waves more aligned with the magnetic field. Thus, we present a mechanism which both heats protons and provides a quasilinear transport of turbulently generated oblique wavevectors toward the parallel direction. We report on our investigation to quantify and compare the effects of these two coupled processes in a coronal hole and in the solar wind.
SH41C-04
Signatures of turbulent heating in heavy-ion velocity distribution functions
The kinetic properties of the solar wind are a result of complex interactions in the solar corona and interplanetary space. Sofar, observations of velocity distribution functions (VDFs) of solar wind heavy ions have been solely 1D. They are known to exhibit non-thermal features , but because they are 1D projections of the 3D velocity phase space it is difficult to interpret them properly. Based on 3D observations of H+ and He2+ from Helios, we have set up a model for heavy-ion VDFs. In it, the magnetic field vector plays a crucial role by defining the symmetry axis of the VDFs. A thermal anisotropy T⊥/T∥≠1 and a beam drifting along the magnetic field vector at a relative speed of approximately the Alfvèn speed are included. The modeled VDFs are analyzed using a virtual detector and then compared with ACE/SWICS data. Our observations give striking evidence for turbulent heating and the existence of heavy-ion beams. The projection of these beams can explain observed differential streaming. We present in-situ measurements at 1 AU and implications for wave-paricle interactions between the Sun and Earth.
SH41C-05
Heating the Solar Wind Through Turbulence and Electron Heat Conduction Modelling
We employ a turbulence transport model to explore the heating of the solar wind by turbulent dissipation, including, for the time, separate equations for heating of electrons and protons. Heating occurs through the deposition of internal energy from kinetic effects that terminate the MHD cascade at small scales. We utilize a simple transport model for three turbulence quantities -- the energy per unit mass, the cross helicity or Alfvénicity, and a similarity length scale. The model includes a von Karman -- Taylor phenomenological model for turbulent dissipation, which modifies the electron and proton temperatures. The involvement of the electron temperature raises several new and interesting issues; How should the electron heat flux be modeled? How long is the collision time between protons and electrons? How much turbulence dissipation goes into heating the electrons and how much goes into heating the protons? Using Voyager and Ulysses observational data, we begin to explore these issues. We find that the inclusion of electron conduction effects provides a more complete description of the solar wind plasma and may help explain the observed temperature profiles.
SH41C-06 INVITED
Turbulence and structures in dispersive MHD
Fluid models including kinetic effects are numerically integrated in one space dimension in order to study the dynamics of randomly driven parallel or kinetic Alfvén waves. Turbulent transfer is strongly inhibited by dispersion, but the intermittent formation of small-scale structures provides an additional channel of energy dissipation. Piecewise power-law spectra are observed, suggesting that small-scale coherent structures could also contribute to the algebraic "dissipation range" of solar wind and magnetosheath turbulence.
SH41C-07
Solar wind turbulence: cascade, dissipation and heating
In this paper we show that a direct evidence for the presence of an inertial energy cascade, the most characteristic signature of hydromagnetic turbulence (MHD), is observed in the solar wind. A Yaglom-like scaling relation has been found in the high-latitude data samples measured by the Ulysses spacecraft. Moreover, an analogous scaling law, suitable modified to take into account compressible fluctuations, has been observed in a much more extended fraction of the same data set. Thus, it seems that large scale density fluctuations, despite their low amplitude, play a major role in the basic scaling properties of turbulence. The presence of a nonlinear turbulent magnetohydrodynamic energy cascade provides for the first time a direct estimation of the turbulent energy transfer rate, which can contribute to the in situ heating of the wind. In particular, The compressive turbulent cascade seems to be able to supply the energy needed to account for the local heating of the non-adiabatic solar wind.