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.

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    ABSTRACT: Supernovae are known to be the dominant energy source for driving turbulence in the interstellar medium. Yet, their effect on magnetic field amplification in spiral galaxies is still poorly understood. Analytical models based on the uncorrelated-ensemble approach predicted that any created field will be expelled from the disk before a significant amplification can occur. By means of direct simulations of supernova-driven turbulence, we demonstrate that this is not the case. Accounting for vertical stratification and galactic differential rotation, we find an exponential amplification of the mean field on timescales of 100Myr. The self-consistent numerical verification of such a "fast dynamo" is highly beneficial in explaining the observed strong magnetic fields in young galaxies. We, furthermore, highlight the importance of rotation in the generation of helicity by showing that a similar mechanism based on Cartesian shear does not lead to a sustained amplification of the mean magnetic field. This finding impressively confirms the classical picture of a dynamo based on cyclonic turbulence. Comment: 99 pages, 46 figures (in part strongly degraded), 8 tables, PhD thesis, University of Potsdam (2009). Resolve URN "urn:nbn:de:kobv:517-opus-29094" (e.g. via for a version with high-resolution figures
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    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.
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    ABSTRACT: We discuss a new type of dust acceleration mechanism that acts in a turbulent magnetized medium. The magnetohydrodynamic turbulence can accelerate grains through resonant as well as non-resonant interactions. We show that the magnetic compression provides higher velocities for super-Alfvénic 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 submicron 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. · 5.52 Impact Factor

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