Wednesday, April 11, 2012

1204.2094 (Andrea Verdini et al.)

Transition from weak to strong cascade in MHD turbulence    [PDF]

Andrea Verdini, Roland Grappin
The transition from weak to strong turbulence when passing from large to small scales in magnetohydrodynamic (MHD) turbulence with guide field is a cornerstone of anisotropic turbulence theory. We present the first check of this transition, using the Shell-RMHD which combines a shell model of perpendicular nonlinear coupling and linear propagation along the guide field. This model allows to reach Reynolds numbers around $10^6$. We obtain surprisingly good agreement with the theoretical predictions, with a reduced perpendicular energy spectrum scaling as $k^{-2}$ at large scales and as $k^{-5/3}$ at small scales, where critical balance between nonlinear and propagation time is reached. However, even in the strong regime, a high level of excitation is found in the weak coupling region of Fourier space, which is due to the rich frequency spectrum of large eddies. A corollary is that the reduced parallel spectral slope is not a definite test of the spectral anisotropy, contrary to standard belief.
View original: http://arxiv.org/abs/1204.2094

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