Structure, complexity and relaxation in turbulence and the solar wind

William H. Matthaeus (Bartol Research Institute and Department of Physics and Astronomy, University of Delaware)

MHD turbulence is often associated with randomness and global processes such as relaxation and cascade, concepts that can meaningfully be explored in the context of solar wind turbulence. Alfvenic turbulence and anisotropy are two familiar examples. However turbulence can also give rise to structure and spatial complexity, which have implications in the behavior of field line random walk and charged particle diffusion. Notable among these are the appearance of "flux tubes" in turbulence and the temporary trapping of charged particles that might explain observed interplanetary dropouts of solar energetic particles. Recent numerical studies of MHD turbulence reveal that structure formation, and therefore spatial intermittency, can be related to local relaxation processes that act rapidly to reduce nonlinear stresses in turbulence. This gives rise to patches of correlations of several types -- force free magnetic field, Beltrami velocity fields, Alfvenic directional alignment and anti-correlations between magnetic and fluid accelerations. These ideas can be applied to a reexamination of MHD discontinuities in the solar wind, and to refinements in our perspective on solar energetic particles transport.