Intermittency of Magnetohydrodynamic Turbulence: Astrophysical Perspective

Intermittency is an essential property of astrophysical fluids, which demonstrate an extended inertial range. As intermittency violates self-similarity of motions, it gets impossible to naively extrapolate the properties of fluid obtained computationally with relatively low resolution to the actual astrophysical situations. In terms of Astrophysics, intermittency affects turbulent heating, momentum transfer, interaction with cosmic rays, radio waves and many more essential processes. Because of its significance, studies of intermittency call for coordinated efforts from both theorists and observers. In terms of theoretical understanding we are still just scratching a surface of a very rich subject. We have some theoretically well justified models that are poorly supported by experiments, we also have She-Leveque model, which could be vulnerable on theoretical grounds, but, nevertheless, is well supported by experimental and laboratory data. I briefly discuss a rather mysterious property of turbulence called ``extended self-similarity'' and the possibilities that it opens for the intermittency research. Then I analyze simulations of MHD intermittency performed by different groups and show that their results do not contradict to each other. Finally, I discuss the intermittency of density, intermittency of turbulence in the viscosity-dominated regime as well as the intermittency of polarization of Alfvenic modes. The latter provides an attractive solution to account for a slower cascading rate that is observed in some of the numerical experiments. I conclude by claiming that a substantial progress in the field may be achieved by studies of the turbulence intermittency via observations.Comment: 14 pages, 7 figures, invited lecture at Trieste, published International Journal of Modern Physics D, July 2