Residual Energy in Magnetohydrodynamic Turbulence (original) (raw)
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Generation of residual energy in the turbulent solar wind
2012
In situ observations of the fluctuating solar wind flow show that the energy of magnetic field fluctuations always exceeds that of the kinetic energy, and therefore the difference between the kinetic and magnetic energies, known as the residual energy, is always negative. The same behaviour is found in numerical simulations of magnetohydrodynamic turbulence. We study the dynamics of the residual energy for strong, anisotropic, critically balanced magnetohydrodynamic turbulence using the eddy damped quasi-normal Markovian approximation. Our analysis shows that for stationary critically balanced magnetohydrodynamic turbulence, negative residual energy will always be generated by nonlinear interacting Alfv en waves. This offers a general explanation for the observation of negative residual energy in solar wind turbulence and in the numerical simulations. V
Identification of Kinetic Alfvén Wave Turbulence in the Solar Wind
The Astrophysical Journal, 2012
The nature of small-scale turbulent fluctuations in the solar wind is investigated using a comparison of Cluster magnetic and electric field measurements to predictions arising from models consisting of either kinetic Alfvén waves or whistler waves. The electric and magnetic field properties of these waves from linear theory are used to construct spacecraft-frame frequency spectra of (|δE|/|δB|) s/c and (|δB |/|δB|) s/c , allowing for a direct comparison to spacecraft data. The measured properties of the small-scale turbulent fluctuations, found to be inconsistent with the whistler wave model, agree well with the prediction of a spectrum of kinetic Alfvén waves with nearly perpendicular wavevectors.
Magnetohydrodynamic Turbulence in the Solar Wind
Annual Review of Astronomy and Astrophysics, 1995
Recent work in describing the solar wind as an MHD turbulent fluid has shown that the magnetic fluctuations are adequately described as time stationary and to some extent as spatially homogeneous. Spectra of the three rugged invariants of incompressible MHD are the principal quantities used to characterize the velocity and magnetic field fluctuations. Unresolved issues concerning the existence of actively developing turbulence are discussed. INTRODUCTION To describe a fluid system as turbulent is to say that the dynamical fluid variables exhibit complex and essentially non-reproducible behavior as a function of time. This is generally due to the presence of nonlinearities in the fluid equations which strongly couple a large number of degrees of freedom. Turbulent systems are usually very far from equilibrium states for which detailed analytically tractable theories might exist. By all appearances, the solar wind plasma flow and the interplanetary magnetic field carried along with it are such a turbulent system. In the zero momentum frame, the magnetic and velocity field fluctuations are energetically comparable to the mean magnetic field over length scales of order I AU and display the type of complicated behavior expected of turbulence. velocity and magnetic fields in magnetohydrodynamie turbulence, EOS,
An application of the turbulent magnetohydrodynamic residual-energy equation model to the solar wind
Physics of Plasmas, 2007
A magnetohydrodynamic ͑MHD͒ turbulence model incorporating the turbulent MHD residual energy ͑difference between the kinetic and magnetic energies͒ is applied to solar-wind turbulence. In the model, the dynamics of the turbulent cross-helicity ͑cross-correlation between the velocity and magnetic field͒ and the turbulent MHD residual energy, which are considered to describe the degree of Alfvénicity of the MHD turbulence, are solved simultaneously with the dynamics of the turbulent MHD energy and its dissipation rate. The transition of solar-wind turbulence from the Alfvén-wave-like fluctuations near the Sun in the inner heliosphere to the fully developed MHD turbulence in the outer heliosphere is discussed. Magnetic dominance in the solar-wind fluctuations is addressed from the dynamics of the evolution equation of the residual energy. An interpretation of the observed Alfvén ratio ͑ratio of the kinetic to magnetic energies͒ of ϳ0.5 is proposed from the viewpoint of a stationary solution of the turbulence model.
Modeling of short scale turbulence in the solar wind
Nonlinear Processes in Geophysics, 2005
The solar wind serves as a laboratory for investigating magnetohydrodynamic turbulence under conditions irreproducible on the terra firma. Here we show that the frame work of Hall magnetohydrodynamics (HMHD), which can support three quadratic invariants and allows nonlinear states to depart fundamentally from the Alfvénic, is capable of reproducing in the inertial range the three branches of the observed solar wind magnetic fluctuation spectrum-the Kolmogorov branch f −5/3 steepening to f −α 1 with α 1 3−4 on the high frequency side and flattening to f −1 on the low frequency side. These fluctuations are found to be associated with the nonlinear Hall-MHD Shear Alfvén waves. The spectrum of the concomitant whistler type fluctuations is very different from the observed one. Perhaps the relatively stronger damping of the whistler fluctuations may cause their unobservability. The issue of equipartition of energy through the so called Alfvén ratio acquires a new status through its dependence, now, on the spatial scale.
Physical Review Letters, 2014
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On the Low-Frequency Boundary of Sun-Generated Magnetohydrodynamic Turbulence in the Slow Solar Wind
The Astrophysical Journal, 2012
New aspects of the slow solar wind turbulent heating and acceleration are investigated. A physical meaning of the lower boundary of the Alfvén wave turbulent spectra in the solar atmosphere and the solar wind is studied and the significance of this natural parameter is demonstrated. Via an analytical and quantitative treatment of the problem we show that a truncation of the wave spectra from the lower frequency side, which is a consequence of the solar magnetic field structure and its cyclic changes, results in a significant reduction of the heat production and acceleration rates. An appropriate analysis is presented regarding the link of the considered problem with existing observational data and slow solar wind initiation scenarios.
Small-Scale Energy Cascade of the Solar Wind Turbulence
Astrophysical Journal, 2008
Magnetic fluctuations in the solar wind are distributed according to Kolmogorov's power law f −5/3 below the ion cyclotron frequency f ci . Above this frequency, the observed steeper power law is usually interpreted in two different ways: a dissipative range of the solar wind turbulence or another turbulent cascade, the nature of which is still an open question. Using the Cluster magnetic data we show that after the spectral break the intermittency increases toward higher frequencies, indicating the presence of non-linear interactions inherent to a new inertial range and not to the dissipative range. At the same time the level of compressible fluctuations raises. We show that the energy transfer rate and intermittency are sensitive to the level of compressibility of the magnetic fluctuations within the small scale inertial range. We conjecture that the time needed to establish this inertial range is shorter than the eddy-turnover time, and is related to dispersive effects. A simple phenomenological model, based on the compressible Hall MHD, predicts the magnetic spectrum ∼ k −7/3+2α , which depends on the degree of plasma compression α.
Imbalanced magnetohydrodynamic turbulence modified by velocity shear in the solar wind
Astrophysics and Space Science
We study incompressible imbalanced magnetohydrodynamic turbulence in the presence of background velocity shears. Using scaling arguments, we show that the turbulent cascade is significantly accelerated when the background velocity shear is stronger than the velocity shears in the subdominant Alfvén waves at the injection scale. The spectral transport is then controlled by the background shear rather than the turbulent shears and the Tchen spectrum with spectral index −1 is formed. This spectrum extends from the injection scale to the scale of the spectral break where the subdominant wave shear becomes equal to the background shear. The estimated spectral breaks and power spectra are in good agreement with those observed in the fast solar wind. The proposed mechanism can contribute to enhanced turbulent cascades and modified −1 spectra observed in the fast solar wind with strong velocity shears. This mechanism can also operate in many other astrophysical environments where turbulence develops on top of non-uniform plasma flows.
Dissipation-scale turbulence in the solar wind
AIP Conference Proceedings, 2007
We present a cascade model for turbulence in weakly collisional plasmas that follows the nonlinear cascade of energy from the large scales of driving in the MHD regime to the small scales of the kinetic Alfvén wave regime where the turbulence is dissipated by kinetic processes. Steady-state solutions of the model for the slow solar wind yield three conclusions: (1) beyond the observed break in the magnetic energy spectrum, one expects an exponential cutoff ; (2) the widely held interpretation that this dissipation range obeys power-law behavior is an artifact of instrumental sensitivity limitations; and, (3) over the range of parameters relevant to the solar wind, the observed variation of dissipation range spectral indices from −2 to −4 is naturally explained by the varying effectiveness of Landau damping, from an undamped prediction of −7/3 to a strongly damped index around −4.