Nonlinear Transverse Cascade and Sustenance of MRI Turbulence in Keplerian Disks with an Azimuthal Magnetic Field
Journal
Date Issued
2017
Author(s)
Abstract
We investigate magnetohydrodynamic turbulence driven by the magnetorotational instability (MRI) in Keplerian
disks with a nonzero net azimuthal magnetic field using shearing box simulations. As distinct from most previous
studies, we analyze turbulence dynamics in Fourier (k-) space to understand its sustenance. The linear growth of MRI
with azimuthal field has a transient character and is anisotropic in Fourier space, leading to anisotropy of nonlinear
processes in Fourier space. As a result, the main nonlinear process appears to be a new type of angular redistribution of
modes in Fourier space – the nonlinear transverse cascade – rather than usual direct/inverse cascade. We demonstrate
that the turbulence is sustained by interplay of the linear transient growth of MRI (which is the only energy supply for
the turbulence) and the transverse cascade. These two processes operate at large length scales, comparable to box size
and the corresponding small wavenumber area, called vital area in Fourier space is crucial for the sustenance, while
outside the vital area direct cascade dominates. The interplay of the linear and nonlinear processes in Fourier space is
generally too intertwined for a vivid schematization. Nevertheless, we reveal the basic subcycle of the sustenance that
clearly shows synergy of these processes in the self-organization of the magnetized flow system. This synergy is quite
robust and persists for the considered different aspect ratios of the simulation boxes. The spectral characteristics of the
dynamical processes in these boxes are qualitatively similar, indicating the universality of the sustenance mechanism
of the MRI-turbulence.
Volume
845
Issue
1
Start page
70
Issn Identifier
0004-637X
Rights
open.access
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