Anisotropy in solar wind plasma turbulence

Abstract

One contribution of 11 to a theme issue 'Dissipation and heating in solar wind turbulence' . A review of spectral anisotropy and variance anisotropy for solar wind fluctuations is given, with the discussion covering inertial range and dissipation range scales. For the inertial range, theory, simulations and observations are more or less in accord, in that fluctuation energy is found to be primarily in modes with quasi-perpendicular wavevectors (relative to a suitably defined mean magnetic field), and also that most of the fluctuation energy is in the vector components transverse to the mean field. Energy transfer in the parallel direction and the energy levels in the parallel components are both relatively weak. In the dissipation range, observations indicate that variance anisotropy tends to decrease towards isotropic levels as the electron gyroradius is approached; spectral anisotropy results are mixed. Evidence for and against wave interpretations and turbulence interpretations of these features will be discussed. We also present new simulation results concerning evolution of variance anisotropy for different classes of initial conditions, each with typical background solar wind parameters.

Third-moment descriptions of the interplanetary turbulent cascade, intermittency and back transfer

“…The more modern view of the K41b versus IK controversy is based upon the orientation of the wave vectors called 'geometry' or 'spectral anisotropy' [57]. If the wave vectors are quasiparallel to the background mean magnetic field, it may be that the IK formalism based on wave dynamics has merit.…”

“…MHD is a fluid approximation to plasma dynamics. Dissipation sets in at the ion inertial scale [43,54,55,57,100], where the fluid approximation breaks down and kinetic physics takes over. However, there is new evidence to suggest that the spectral break may occur at the proton Larmor radius [101,102] under some conditions.…”

“…Going beyond intermittency, we will show that there generally exists an anti-correlation between the energy cascade of the outward-and inward-propagating components of the solar wind fluctuations that is a fundamental aspect of the turbulence and we will attempt to explain this anti-correlation in terms of the large-scale energy-containing fluctuations. The reader is encouraged to read several particularly relevant papers in this same issue [43,[53][54][55][56][57].…”

Dynamic properties of small-scale solar wind plasma fluctuations

“…the reviews [1,2], reviews in the current issue [3][4][5][6][7] and references therein). The main aim of these investigations is to understand the general process of energy exchange in the course of nonlinear interaction between the structures of different time and spatial scales.…”

“…The authors found that the probability distribution functions (PDFs) of the fluctuations are highly non-Gaussian over this range, but they do not show large changes in shape with scale, statistically similar to those of the magnetic fluctuations. We also need to take into account the strong anisotropy of turbulence in the SW (see [7,[28][29][30][31]), which can finally lead to different deviations from the Kolmogorov model often observed in experiments. Here, we present some new results concerning turbulent properties of small-scale SW ion flux fluctuations on the basis of the SPECTR-R measurements with the record high time resolution up to 32 Hz.…”