Scaling, Intermittency and Decay of MHD Turbulence

We discuss a few recent developments that are important for understanding ofMHD turbulence. First, MHD turbulence is not so messy as it is usually believed. In fact, thenotion of strong non-linear coupling of compressible and incompressible motionsalong MHD cascade is not tenable. Alfven, slow and fast modes of MHD turbulencefollow their own cascades and exhibit degrees of anisotropy consistent withtheoretical expectations. Second, the fast decay of turbulence is not relatedto the compressibility of fluid. Rates of decay of compressible andincompressible motions are very similar. Third, viscosity by neutrals does notsuppress MHD turbulence in a partially ionized gas. Instead, MHD turbulencedevelops magnetic cascade at scales below the scale at which neutrals dampordinary hydrodynamic motions. Forth, density statistics does not exhibit theuniversality that the velocity and magnetic field do. For instance, at smallMach numbers the density is anisotropic, but it gets isotropic at high Machnumbers. Fifth, the intermittency of magnetic field and velocity are different.Both depend on whether the measurements are done in local system of referenceoriented along the local magnetic field or in the global system of referencerelated to the mean magnetic field.Comment: 12 pages, Invited Review, Workshop on Theoretical Plasma Physics, Trieste, Italy, 5-16 Jul