Magnetohydrodynamic Turbulence in the Solar Wind (original) (raw)

The Solar Wind as a Turbulence Laboratory

Living Reviews in Solar Physics, 2013

In this review we will focus on a topic of fundamental importance for both plasma physics and astrophysics, namely the occurrence of large-amplitude low-frequency fluctuations of the fields that describe the plasma state. This subject will be treated within the context of the expanding solar wind and the most meaningful advances in this research field will be reported emphasizing the results obtained in the past decade or so. As a matter of fact, Ulysses' high latitude observations and new numerical approaches to the problem, based on the dynamics of complex systems, brought new important insights which helped to better understand how turbulent fluctuations behave in the solar wind. In particular, numerical simulations within the realm of magnetohydrodynamic (MHD) turbulence theory unraveled what kind of physical mechanisms are at the basis of turbulence generation and energy transfer across the spectral domain of the fluctuations. In other words, the advances reached in these past years in the investigation of solar wind turbulence now offer a rather complete picture of the phenomenological aspect of the problem to be tentatively presented in a rather organic way.

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.

MHD Structures, Waves and Turbulence in the Solar Wind: Observations and Theories

Physics Today, 1996

A comprehensive overview is presented of recent observational and theoretical results on solar wind structures and fluctuations and magnetohydrodynamic waves and turbulence, with preference given to phenomena in the inner heliosphere. Emphasis is placed on the progress made in the past decade in the understanding of the nature and origin of especially small-scale, compressible and incompressible fluctuations. Turbulence models to describe the spatial transport and spectral transfer of the fluctuations in the inner heliosphere are discussed, and results from direct numerical simulations are dealt with. Intermittency of solar wind fluctuations and their statistical distributions are briefly investigated. Studies of the heating and acceleration effects of the turbulence on the background wind are critically surveyed. Finally, open questions concerning the origin, nature and evolution of the fluctuations are listed, and possible avenues and perspectives for future research are outlined.

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.

Scaling Laws of Turbulence and Heating of Fast Solar Wind: The Role of Density Fluctuations

Physical Review Letters, 2009

Incompressible and isotropic magnetohydrodynamic turbulence in plasmas can be described by an exact relation for the energy flux through the scales. This Yaglom-like scaling law has been recently observed in the solar wind above the solar poles observed by the Ulysses spacecraft, where the turbulence is in an Alfvénic state. An analogous phenomenological scaling law, suitably modified to take into account compressible fluctuations, is observed more frequently in the same dataset. Large scale density fluctuations, despite their low amplitude, play thus a crucial role in the basic scaling properties of turbulence. The turbulent cascade rate in the compressive case can moreover supply the energy dissipation needed to account for the local heating of the non-adiabatic solar wind. PACS numbers: 96.50.Ci; 47.27.Gs; 96.50.Tf; 52.35.Ra The interplanetary space is permeated by the solar wind [1], a magnetized, supersonic flow of charged particles originating in the high solar atmosphere and blowing away from the sun. Low frequency fluctuations of solar wind variables are often described in the framework of fully developed hydromagnetic (MHD) turbulence . The large range of scales involved, spanning from 1 AU (≃ 1.5 × 10 8 km) down to a few kilometers, make the solar wind the largest "laboratory" where MHD turbulence can be investigated using measurements collected in situ by instruments onboard spacecraft . MHD turbulence is often investigated through the Elsässer variables z ± = v ± (4πρ) −1/2 b, computed from the local plasma velocity v and magnetic field b, ρ being the plasma mass density. In terms of such variables, MHD equations can be rewritten as ∂ t z ± + z ∓ · ∇z ± = −∇P/ρ + diss, where P is the total hydromagnetic pressure, and diss indicates dissipative terms involving the viscosity and the magnetic diffusivity. As in the Navier-Stokes equations for neutral fluids, the nonlinear terms z ∓ · ∇z ± cause the turbulent energy transfer between different scales, at high Reynolds numbers where dissipative terms can be neglected. However, in the MHD case, they couple the two Elsässer variables, so that the Alfvénic MHD fluctuations z ± , propagating along the background magnetic field, are advected by fluctuations z ∓ propagating in the opposite direction. The presence of strong correlations (or anti-correlations) between velocity and magnetic fluctuations, along with a nearly constant magnetic intensity and low amplitude density fluctuations, is usually referred to as Alfvénic state of turbulence, and implies that one of the two modes z ± should be negligible, making the nonlinear term of MHD equations vanish for pure Alfvénic fluctuations. In that case, the turbulent energy transfer should also disappear . Alfvénic turbulence is observed almost ubiquitously in fast wind. This holds both in the ecliptic fast streams, and in the high latitude wind blowing directly from the sun coronal holes .

Magnetically dominated structures as an important component of the solar wind turbulence

Annales Geophysicae, 2007

This study focuses on the role that magnetically dominated fluctuations have within the solar wind MHD turbulence. It is well known that, as the wind expands, magnetic energy starts to dominate over kinetic energy but we lack of a statistical study apt to estimate the relevance of these fluctuations depending on wind speed, radial distance from the sun and heliographic latitude. Our results suggest that this kind of fluctuations can be interpreted as non-propagating structures, advected by the wind during its expansion. In particular, observations performed in the ecliptic revealed a clear radial dependence of these magnetic structures within fast wind, but not within slow wind. At short heliocentric distances (~0.3 AU) the turbulent population is largely dominated by Alfvénic fluctuations characterized by high values of normalized cross-helicity and a remarkable level of energy equipartition. However, as the wind expands, a new-born population, characterized by lower values of Alfvénicity and a clear imbalance in favor of magnetic energy becomes visible and clearly distinguishable from the Alfvénic population largely characterized by an outward sense of propagation. We estimate that more than 20% of all the analyzed intervals of hourly scale within fast wind are characterized by normalized cross-helicity close to zero and magnetic energy largely dominating over kinetic energy. Most of these advected magnetic structures result to be non-compressive and might represent the crossing of the border between adjacent flux tubes forming, as suggested in literature, the advected background structure of the interplanetary magnetic field. On the other hand, their features are also well fitted by the Magnetic Field Directional Turnings paradigm as proposed in literature.

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.

Magnetic fluctuations and Hall magnetohydrodynamic turbulence in the solar wind

Journal of Geophysical Research, 2004

It is shown that the framework of Hall magnetohydrodynamics (Hall-MHD), 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 À5/3 , steepening to f Àa 1 , with a 1 ' 3-4 on the high-frequency side and flattening to f À1 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 the anisotropy of the turbulence is addressed briefly.

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.

Radial Variations of Large-Scale Magnetohydrodynamic Fluctuations in the Solar Wind

Journal of Geophysical Research, 1984

Two time periods are studied for gomery, 1983) of interpreting solar wind observa- which a relatively complete set of magnetic field tions in terms of turbulence theory. MHD turbu- and plasma measurements are available at both 1 AU lence involves a dynamical transfer of energy (from IMP 8 and ISEE 3) and near 4 to 5 AU (from between various