TY - JOUR

T1 - Zero Net Flux MRI Turbulence in Disks

T2 - Sustenance Scheme and Magnetic Prandtl Number Dependence

AU - Mamatsashvili, George

AU - Chagelishvili, George

AU - Pessah, Martin E.

AU - Stefani, Frank

AU - Bodo, Gianluigi

PY - 2020/11/20

Y1 - 2020/11/20

N2 - We investigate sustenance and dependence on magnetic Prandtl number (Pm) for magnetorotational instability (MRI)-driven turbulence in Keplerian disks with zero net magnetic flux using standard shearing box simulations. We focus on the turbulence dynamics in Fourier space, capturing specific/noncanonical anisotropy of nonlinear processes due to disk flow shear. This is a new type of nonlinear redistribution of modes over wavevector orientations in Fourier space-the nonlinear transverse cascade-which is generic to shear flows and fundamentally different from the usual direct/inverse cascade. The zero flux MRI has no exponentially growing modes, so its growth is transient, or nonmodal. Turbulence self-sustenance is governed by constructive cooperation of the transient growth of MRI and the nonlinear transverse cascade. This cooperation takes place at small wavenumbers (on the flow size scales) referred to as the vital area in Fourier space. The direct cascade transfers mode energy from the vital area to larger wavenumbers. At large Pm, the transverse cascade prevails over the direct one, keeping most of modes' energy contained in small wavenumbers. With decreasing Pm, however, the action of the transverse cascade weakens and can no longer oppose the action of the direct cascade, which more efficiently transfers energy to higher wavenumbers, leading to increased resistive dissipation. This undermines the sustenance scheme, resulting in the turbulence decay. Thus, the decay of zero net flux MRI turbulence with decreasing Pm is attributed to the topological rearrangement of the nonlinear processes when the direct cascade begins to prevail over the transverse cascade.

AB - We investigate sustenance and dependence on magnetic Prandtl number (Pm) for magnetorotational instability (MRI)-driven turbulence in Keplerian disks with zero net magnetic flux using standard shearing box simulations. We focus on the turbulence dynamics in Fourier space, capturing specific/noncanonical anisotropy of nonlinear processes due to disk flow shear. This is a new type of nonlinear redistribution of modes over wavevector orientations in Fourier space-the nonlinear transverse cascade-which is generic to shear flows and fundamentally different from the usual direct/inverse cascade. The zero flux MRI has no exponentially growing modes, so its growth is transient, or nonmodal. Turbulence self-sustenance is governed by constructive cooperation of the transient growth of MRI and the nonlinear transverse cascade. This cooperation takes place at small wavenumbers (on the flow size scales) referred to as the vital area in Fourier space. The direct cascade transfers mode energy from the vital area to larger wavenumbers. At large Pm, the transverse cascade prevails over the direct one, keeping most of modes' energy contained in small wavenumbers. With decreasing Pm, however, the action of the transverse cascade weakens and can no longer oppose the action of the direct cascade, which more efficiently transfers energy to higher wavenumbers, leading to increased resistive dissipation. This undermines the sustenance scheme, resulting in the turbulence decay. Thus, the decay of zero net flux MRI turbulence with decreasing Pm is attributed to the topological rearrangement of the nonlinear processes when the direct cascade begins to prevail over the transverse cascade.

KW - Stellar accretion disks

KW - Magnetohydrodynamics

KW - Plasma astrophysics

KW - High energy astrophysics

KW - Protoplanetary disks

KW - Magnetic fields

KW - Interplanetary turbulence

KW - ANGULAR-MOMENTUM TRANSPORT

KW - LOCAL SHEAR INSTABILITY

KW - MAGNETOROTATIONAL INSTABILITY

KW - TRANSIENT GROWTH

KW - ACCRETION DISKS

KW - MHD SIMULATIONS

KW - FLOWS

KW - SATURATION

KW - BOX

KW - PERTURBATIONS

U2 - 10.3847/1538-4357/abbd42

DO - 10.3847/1538-4357/abbd42

M3 - Journal article

VL - 904

JO - Astrophysical Journal

JF - Astrophysical Journal

SN - 0004-637X

IS - 1

M1 - 47

ER -