Physics – Plasma Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsh53a..01m&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SH53A-01
Physics
Plasma Physics
[2149] Interplanetary Physics / Mhd Waves And Turbulence, [2159] Interplanetary Physics / Plasma Waves And Turbulence, [4490] Nonlinear Geophysics / Turbulence, [7863] Space Plasma Physics / Turbulence
Scientific paper
“Universality” in hydrodynamic turbulence usually refers to the existence of unique statistical distributions that are obtained as the Reynolds number tends to infinity. In such cases the hierarchy of structure functions would approach particular functional forms, implying for example a standard form of the energy spectrum. Magnetohydrodynamics (MHD) is more complex for several reasons. First, there are several spectral fluxes and several types of cascades, including inverse and direct cascades. Second, there are several additional dimensionless parameters, including the ratio of magnetic Reynolds number to Reynolds number, the ratio of kinetic to magnetic energy, and when cross helicity is nonzero, the ratio of two fundamental lengths scales - one for each Elsasser field. For these reasons it is doubtful that there are universal MHD turbulence statistics, in the usual sense. Recent simulation work supports this conclusion, as very similar initial energy spectra can lead to very different time evolution depending on details of initial conditions. We suggest here another approach to finding unifying principles for MHD turbulence. We begin with discussion of spontaneous generation of distinctive local correlations, including those corresponding to Beltrami, Alfvenic and force-free states. An argument is outlined as to how these correlations require certain higher order correlations to be generated. In real space these correspond to generation of region of reduced nonlinearity. Spatial patches of correlation may be separated by quasi-discontinuous jumps in the local parameters. This gives rise, e.g., to frequent appearance of “tangential discontinuities” due to formation of current sheets (and the possibility of reconnection) between adjacent quasi-relaxed flux tubes. Recent evidence supports the view that (1) the generation of high wavenumber nonGaussian statistics is a signature of these local rapid relaxation processes, (2) distinctive correlations form in patches, producing a “cellular” structure (3) local relaxation has a random character, e.g., random Alfvenic alignments in the solar wind, (4) that large local increment variances are good indicators of the coherent structures that are responsible for dynamically produced nonGaussianity, and finally (5) the coherent strictures formed in this way are associated with enhanced dissipation and heating in simulations and in the solar wind. This view of turbulence cannot be captured by looking only at spectral laws, and provides a real space picture of intermittency. We argue that the key to understanding the diversity of MHD behavior lies in understanding the higher order correlations that regulate local relaxation processes. Understanding these higher order correlations and their spatial manifestations might provide a path towards a unified description of the many types of MHD turbulence.
Dmitruk Pablo
Greco Andrés
Matthaeus William H.
Osman K.
Oughton Sean
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