We propose a phenomenological theory of strong incompressible magnetohydrodynamic turbulence in the presence large-scale external magnetic field. argue that inertial range scales, magnetic-field and velocity-field fluctuations tend to align directions their polarizations. However, perfect alignment cannot be reached, it is precluded by constant energy flux over scales. As consequence, fluid at each scale $\lambda$ become effectively aligned within angle $\phi_{\lambda}\propto \lambda^{1/4}$,...

The nature of subproton scale fluctuations in the solar wind is an open question, partly because two similar types electromagnetic turbulence can occur: kinetic Alfven and whistler turbulence. These possibilities, however, have one key qualitative difference: turbulence, unlike has negligible power density fluctuations. In this Letter, we present new observational data, as well analytical numerical results, to investigate difference. results show, for first time, that below proton are...

We propose a phenomenological model for incompressible magnetohydrodynamic turbulence. argue that nonlinear-wave interaction weakens as the energy cascade proceeds to small scales; however, anisotropy of fluctuations along large-scale magnetic field increases, which makes turbulence strong at all scales. To explain weakening interaction, we small-scale velocity and fields become increasingly dynamically aligned their scale decreases, so turbulent "eddies" locally anisotropic in plane...

Motivated by recent analytic predictions, we report numerical evidence showing that in driven incompressible magnetohydrodynamic turbulence the magnetic- and velocity-field fluctuations locally tend to align directions of their polarizations. This dynamic alignment is stronger at smaller scales with angular mismatch between polarizations decreasing scale \lambda approximately as \theta_\lambda ~ \lambda^{1/4}. can naturally lead a weakening nonlinear interactions provide an explanation for...

The power spectrum of magnetic fluctuations in the solar wind at 1 AU displays a break between two laws range spacecraft-frame frequencies 0.1 to Hz. These correspond spatial scales plasma frame near proton gyroradius ρi and inertial length di. At it is difficult determine which these associated with break, since [Formula: see text] perpendicular ion beta typically β⊥i∼1. To address this, several exceptional intervals β⊥i≪1 β⊥i≫1 were investigated, during well separated. It was found that...

A numerical study of strong kinetic-Alfvén turbulence at scales smaller than the ion gyroscale is presented, and a phenomenological model proposed that argues magnetic density fluctuations are concentrated mostly in two-dimensional structures, which leads to their Fourier energy spectra E(k⊥)∝k−8/3⊥, where k⊥ wavevector component normal background field. The results may provide an explanation for recent observations solar wind sub-proton scales.

We present an analytical study of subproton electromagnetic fluctuations in a collisionless plasma with beta the order unity. In linear limit, rigorous derivation from kinetic equation is conducted focusing on role and physical properties kinetic-Alfvén whistler waves. Then, nonlinear fluid-like equations for waves modes are derived, special emphasis similarities differences corresponding dynamics. The exist lower-frequency region phase space, ω ≪ k⊥vTi, where they described by system. These...

We develop a framework for studying the statistical properties of current sheets in numerical simulations magnetohydrodynamic (MHD) turbulence with strong guide field, as modeled by reduced MHD. describe an algorithm that identifies simulation snapshot and then determines their geometrical (including length, width, thickness) intensities (peak density total energy dissipation rate). apply this procedure to MHD perform analysis on obtained population sheets. evaluate role reconnection...

The current understanding of magnetohydrodynamic (MHD) turbulence envisions turbulent eddies which are anisotropic in all three directions. In the plane perpendicular to local mean magnetic field, this implies that such become current-sheetlike structures at small scales. We analyze role reconnection these and conclude becomes important a scale $\ensuremath{\lambda}\ensuremath{\sim}L{S}_{L}^{\ensuremath{-}4/7}$, where ${S}_{L}$ is outer-scale ($L$) Lundquist number $\ensuremath{\lambda}$...

We analyze the initial, kinematic stage of magnetic field evolution in an isotropic and homogeneous turbulent conducting fluid with a rough velocity field, $v(l)\ensuremath{\sim}{l}^{\ensuremath{\alpha}}$, $\ensuremath{\alpha}<1$. This regime is relevant to problem generation fluids small Prandtl number, i.e., Ohmic resistivity much larger than viscosity. propose that smaller roughness exponent $\ensuremath{\alpha}$, Reynolds number needed excite fluctuations. implies numerical or...

Strong incompressible three-dimensional magnetohydrodynamic turbulence is investigated by means of high-resolution direct numerical simulations. The simulations show that the configuration space characterized regions positive and negative cross-helicity, corresponding to highly aligned or antialigned velocity magnetic field fluctuations, even when average cross-helicity zero. To elucidate role spectra structure are obtained in ``imbalanced'' where nonzero. When averaged over result...

The question is addressed as to what extent incompressible magnetohydrodynamics can describe random magnetic and velocity fluctuations measured in the solar wind. It demonstrated that distributions of spectral indices for velocity, field, total energy obtained from high-resolution numerical simulations magnetohydrodynamic turbulence are qualitatively quantitatively similar wind observations at 1 AU. Both show inertial range field spectrum Eb steeper than Ev with ≳ magnitude residual ER = −...

The energy spectrum of magnetohydrodynamic turbulence attracts interest due to its fundamental importance and relevance for interpreting astrophysical data. Here we present measurements the spectra from a series high-resolution direct numerical simulations magnetohydrodynamics with strong guide field increasing Reynolds number. presented simulations, resolutions up 20483 mesh points statistics accumulated over 30 150 eddy turnover times, constitute, best our knowledge, largest statistical...

We present a combined observational and theoretical analysis to investigate the nature of plasma turbulence at kinetic scales in Earth's magnetosheath. In first decade range, just below ion gyroscale, was found be similar that upstream solar wind: predominantly anisotropic, low-frequency Alfvén nature. A key difference, however, is magnetosheath ions are typically much hotter than electrons, $T_\mathrm{i}\gg T_\mathrm{e}$, which, together with $β_\mathrm{i}\sim 1$, leads change behaviour...

We present a direct numerical and analytical study of driven supersonic MHD turbulence that is believed to govern the dynamics star-forming molecular clouds. describe statistical properties by measuring velocity difference structure functions up fifth order. In particular, power spectrum in inertial range found be close E(k) \~ k^{-1.74}, scales as ~ L^{0.42}. The results agree well with Kolmogorov--Burgers model suggested for [astro-ph/0108300]. then generalize more realistic, fractal...

The process of star formation in interstellar molecular clouds is believed to be controlled by driven supersonic magnetohydrodynamic turbulence. We suggest that the inertial range such turbulence obeys Kolmogorov law, while dissipative it behaves as Burgers developing shock singularities. On base She-Leveque analytical model we then predict velocity power spectrum E_k ~ k^{-1.74}. This result reproduces observational Larson l^{0.74...0.76}, [Larson, MNRAS 194 (1981) 809] and agrees well with...

We report the results of an extensive set direct numerical simulations forced, incompressible, magnetohydrodynamic (MHD) turbulence with a strong guide field. The aim is to resolve controversy regarding power-law exponent ($\ensuremath{\alpha}$, say) field-perpendicular energy spectrum $E({k}_{\ensuremath{\perp}})\ensuremath{\propto}{k}_{\ensuremath{\perp}}^{\ensuremath{\alpha}}$. two main theoretical predictions $\ensuremath{\alpha}=\ensuremath{-}3/2$ and...

Abstract Magnetic field fluctuations in magnetohydrodynamic turbulence can be viewed as current sheets that are progressively more anisotropic at smaller scales. As suggested by Loureiro &amp; Boldyrev and Mallet et al., below a certain critical thickness, <?CDATA ${\lambda }_{c}$?> , such become tearing-unstable. We propose the tearing instability changes effective alignment of magnetic lines way to balance eddy turnover rate all scales than . result, turbulent less scales, with angle...

Abstract In astrophysical relativistic plasmas, Alfvénic turbulence exists across a wide range of scales. Similar to nonrelativistic case, this can be influenced by the tearing instability. We argue that in an ultrarelativistic pair plasma effects become significant at critical scale given <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>a</mml:mi> <mml:mi>t</mml:mi> </mml:msub> <mml:mo>∼</mml:mo> <mml:mi>d</mml:mi> <mml:mi>rel</mml:mi>...

Abstract Alfvénic interactions that transfer energy from large to small spatial scales lie at the heart of magnetohydrodynamic turbulence. An important feature turbulence is generation negative residual energy—excess in magnetic fluctuations compared velocity fluctuations. By contrast, an MHD Alfvén wave has equal amounts each type. quasi-modes do not satisfy dispersion relation and exist only presence a nonlinear term can contain either positive or energy, but until now, intuitive physical...

The existence of a weak galactic magnetic field has been repeatedly confirmed by observational data.The origin this not as yet explained in fully satisfactory way and represents one the main challenges astrophysical dynamo theory.In both theory primordial-origin theory, major influence is exerted small-scale fluctuations.This article devoted to constructing systematic second-order statistical such fields.The statistics these fields are studied kinematic approximation for case large Prandtl...

Supersonic turbulent flows of magnetized gas are believed to play an important role in the dynamics star-forming clouds galaxies. Understanding statistical properties such is crucial for developing a theory star formation. In this Letter we propose unified approach obtaining velocity scaling compressible and super-Alfvénic turbulence, valid arbitrary sonic Mach number, M(S). We demonstrate with numerical simulations that can be described She-Lévêque formalism, where only one parameter,...

The interstellar medium provides a unique laboratory for highly supersonic, driven hydrodynamic turbulence. We propose theory of such turbulence, test it by numerical simulations, and use the results to explain observational scaling properties molecular clouds, regions where stars are born.

Recent measurements of solar wind turbulence report the presence intermittent, exponentially distributed angular discontinuities in magnetic field. In this Letter, we study whether such can be produced by magnetohydrodynamic (MHD) turbulence. We detect measuring fluctuations field direction, Δθ, across fixed spatial increments Δx direct numerical simulations MHD with an imposed uniform guide B(0). A large region probability density function (pdf) for Δθ is found to follow exponential decay,...

We develop a model for the strahl population in solar wind – narrow, low-density and high-energy electron beam centred on magnetic field direction. Our is based solution of drift-kinetic equation at heliospheric distances where plasma density, temperature strength decline as power laws distance along flux tube. depends number parameters that, absence analytic full velocity distribution function (eVDF), cannot be derived from theory. however demonstrate that these can efficiently found...