Chapter

MHD Turbulence: Scaling Laws and Astrophysical Implications

DOI: 10.1007/3-540-36238-X_3

ABSTRACT Turbulence is the most common state of astrophysical flows. In typical astrophysical fluids, turbulence is accompanied by
strong magnetic fields, which has a large impact on the dynamics of the turbulent cascade. Recently, there has been a significant
breakthrough on the theory of magnetohydrodynamic (MHD) turbulence. For the first time we have a scaling model that is supported
by both observations and numerical simulations. We review recent progress in studies of both incompressible and compressible
turbulence. We compare Iroshnikov-Kraichnan and Goldreich-Sridhar models, and discuss scalings of Alfvén, slow, and fast waves.
We also discuss the completely new regime of MHD turbulence that happens below the scale at which hydrodynamic turbulent motions
are damped by viscosity. In the case of the partially ionized diffuse interstellar gas the viscosity is due to neutrals and
truncates the turbulent cascade at ~parsec scales. We show that below this scale magnetic fluctuations with a shallow spectrum
persist and discuss the possibility of a resumption of the MHD cascade after ions and neutrals decouple. We discuss the implications
of this new insight into MHD turbulence for cosmic ray transport, grain dynamics, etc., and how to test theoretical predictions
against observations.

Full-text

Available from: A. Lazarian, Dec 31, 2013
0 Followers
 · 
76 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We discuss a new type of dust acceleration mechanism that acts in a turbulent magnetized medium. The magnetohydrodynamic (MHD) turbulence can accelerate grains through resonant as well as nonresonant interactions. We show that the magnetic compression provides higher velocities for super-Alfvenic turbulence and can accelerate an extended range of grains in warm media compared to gyroresonance. While fast modes dominate the acceleration for the large grains, slow modes can be important for sub-micron grains. We provide comprehensive discussion of all the possible grain acceleration mechanisms in interstellar medium. We show that supersonic velocities are attainable for Galactic dust grains. We discuss the consequence of the acceleration. The implications for extinction curve, grain alignment, chemical abundance, etc, are provided.
    Monthly Notices of the Royal Astronomical Society 07/2009; 397(2):1093 - 1100. DOI:10.1111/j.1365-2966.2009.15070.x · 5.23 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present numerical simulations and explore scalings and anisotropy of compressible magnetohydrodynamic (MHD) turbulence. Our study covers both gas-pressure-dominated (high β) and magnetic-pressure-dominated (low β) plasmas at different Mach numbers. In addition, we present results for super-Alfvénic turbulence and discuss in what way it is similar to sub-Alfvénic turbulence. We describe a technique of separating different magnetohydrodynamic modes (slow, fast and Alfvén) and apply it to our simulations. We show that, for both high- and low-β cases, Alfvén and slow modes reveal a Kolmogorov k−5/3 spectrum and scale-dependent Goldreich–Sridhar anisotropy, while fast modes exhibit a k−3/2 spectrum and isotropy. We discuss the statistics of density fluctuations arising from MHD turbulence in different regimes. Our findings entail numerous astrophysical implications ranging from cosmic ray propagation to gamma ray bursts and star formation. In particular, we show that the rapid decay of turbulence reported by earlier researchers is not related to compressibility and mode coupling in MHD turbulence. In addition, we show that magnetic field enhancements and density enhancements are marginally correlated. Addressing the density structure of partially ionized interstellar gas on astronomical-unit scales, we show that the viscosity-damped regime of MHD turbulence that we reported earlier for incompressible flows persists for compressible turbulence and therefore may provide an explanation for these mysterious structures.
    Monthly Notices of the Royal Astronomical Society 09/2003; 345(1):325 - 339. DOI:10.1046/j.1365-8711.2003.06941.x · 5.23 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Astrophysical turbulence is magnetohydrodynamic (MHD) in nature. We discuss fundamental properties of MHD turbulence and in particular the generation of compressible MHD waves by Alfvnic turbulence and show that this process is inefficient. This allows us to study the evolution of different types of MHD perturbations separately. We describe how to separate MHD fluctuations into three distinct families: Alfvn, slow, and fast modes. We find that the degree of suppression of slow and fast modes production by Alfvnic turbulence depends on the strength of the mean field. We review the scaling relations of the modes in strong MHD turbulence. We show that Alfvn modes in compressible regime exhibit scalings and anisotropy similar to those in incompressible regime. Slow modes passively mimic Alfvn modes. However, fast modes exhibit isotropy and a scaling similar to that of acoustic turbulence both in high and low plasmas. We show that our findings entail important consequences for star formation theories, cosmic ray propagation, dust dynamics, and gamma ray bursts. We anticipate many more applications of the new insight to MHD turbulence and expect more revisions of the existing paradigms of astrophysical processes as the field matures.
    Theoretical and Computational Fluid Dynamics 04/2005; 19(2):127-157. DOI:10.1007/s00162-004-0157-x · 1.75 Impact Factor