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Title: ON THE ANISOTROPIC NATURE OF MRI-DRIVEN TURBULENCE IN ASTROPHYSICAL DISKS

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 temporalmore » 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.« less

Authors:
;  [1]
  1. Niels Bohr International Academy, Niels Bohr Institute, Blegdamsvej 17, 2100 Copenhagen (Denmark)
Publication Date:
OSTI Identifier:
22522519
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 802; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCRETION DISKS; ANGULAR MOMENTUM; ANISOTROPY; ASTROPHYSICS; BLACK HOLES; COMPARATIVE EVALUATIONS; FLUCTUATIONS; MAGNETOHYDRODYNAMICS; NMR IMAGING; PERIODICITY; POTENTIALS; SPACE; TENSORS; TURBULENCE; TURBULENT FLOW

Citation Formats

Murphy, Gareth C., and Pessah, Martin E., E-mail: gmurphy@nbi.dk, E-mail: mpessah@nbi.dk. ON THE ANISOTROPIC NATURE OF MRI-DRIVEN TURBULENCE IN ASTROPHYSICAL DISKS. United States: N. p., 2015. Web. doi:10.1088/0004-637X/802/2/139.
Murphy, Gareth C., & Pessah, Martin E., E-mail: gmurphy@nbi.dk, E-mail: mpessah@nbi.dk. ON THE ANISOTROPIC NATURE OF MRI-DRIVEN TURBULENCE IN ASTROPHYSICAL DISKS. United States. doi:10.1088/0004-637X/802/2/139.
Murphy, Gareth C., and Pessah, Martin E., E-mail: gmurphy@nbi.dk, E-mail: mpessah@nbi.dk. Wed . "ON THE ANISOTROPIC NATURE OF MRI-DRIVEN TURBULENCE IN ASTROPHYSICAL DISKS". United States. doi:10.1088/0004-637X/802/2/139.
@article{osti_22522519,
title = {ON THE ANISOTROPIC NATURE OF MRI-DRIVEN TURBULENCE IN ASTROPHYSICAL DISKS},
author = {Murphy, Gareth C. and Pessah, Martin E., E-mail: gmurphy@nbi.dk, E-mail: mpessah@nbi.dk},
abstractNote = {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.},
doi = {10.1088/0004-637X/802/2/139},
journal = {Astrophysical Journal},
number = 2,
volume = 802,
place = {United States},
year = {Wed Apr 01 00:00:00 EDT 2015},
month = {Wed Apr 01 00:00:00 EDT 2015}
}