Abstract
The magnetorotational instability (MRI) is thought to play an important
role in enabling accretion in sufficiently ionized astrophysical disks.
The rate at which MRI-driven turbulence transports angular momentum is
intimately related to both the strength of the amplitudes of the
fluctuations on various scales and the degree of anisotropy of the
underlying turbulence. This has motivated several studies to
characterize the distribution of turbulent power in spectral space. In
this paper we investigate the anisotropic nature of MRI-driven
turbulence using a pseudo-spectral code and introduce novel ways for
providing a robust characterization of the underlying turbulence. We
study the growth of the MRI and the subsequent transition to turbulence
via parasitic instabilities, identifying their potential signature in
the late linear stage. We show that the general flow properties vary in
a quasi-periodic way on timescales comparable to ∼10 inverse angular
frequencies, motivating the temporal analysis of its anisotropy. We
introduce a 3D tensor invariant analysis to quantify and classify the
evolution of the anisotropy of the turbulent flow. This analysis shows a
continuous high level of anisotropy, with brief sporadic transitions
toward two- and three-component isotropic turbulent flow. This
temporal-dependent anisotropy renders standard shell averaging
especially when used simultaneously with long temporal averages,
inadequate for characterizing MRI-driven turbulence. We propose an
alternative way to extract spectral information from the turbulent
magnetized flow, whose anisotropic character depends strongly on time.
This consists of stacking 1D Fourier spectra along three orthogonal
directions that exhibit maximum anisotropy in Fourier space. The
resulting averaged spectra show that the power along each of the three
independent directions differs by several orders of magnitude over most
scales, except the largest ones. Our results suggest that a
first-principles theory to describe fully developed MRI-driven
turbulence will likely have to consider the anisotropic nature of the
flow at a fundamental level.
Originalsprog | Engelsk |
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Artikelnummer | 139 |
Tidsskrift | Astrophysical Journal |
Vol/bind | 802 |
Udgave nummer | 2 |
ISSN | 0004-637X |
DOI | |
Status | Udgivet - 1 apr. 2015 |