Zero net flux MRI-turbulence in disks $-$ sustenance scheme and magnetic Prandtl number dependence

TL;DR

This paper explores how magnetic Prandtl number influences MRI-driven turbulence in Keplerian disks, revealing a new nonlinear mode redistribution process and its impact on turbulence sustenance and decay.

Contribution

It introduces the concept of nonlinear transverse cascade in Fourier space and demonstrates its role in MRI turbulence sustenance and decay depending on Pm.

Findings

High Pm favors turbulence sustenance via transverse cascade.

Decreasing Pm shifts energy transfer to higher wavenumbers, leading to turbulence decay.

The nonlinear transverse cascade is a key process in shear flow turbulence dynamics.

Abstract

We investigate sustenance and dependence on magnetic Prandtl number (${\rm Pm}$) for magnetorotational instability (MRI)-driven turbulence in astrophysical 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 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 ${\rm Pm}$, the transverse cascade prevails over the direct one, keeping most of modes' energy contained in small wavenumbers. With decreasing ${\rm Pm}$, however, the action of the transverse cascade weakens and can no longer oppose the action of 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 ${\rm Pm}$ is attributed to topological rearrangement of the nonlinear processes when the direct cascade begins to prevail over the transverse cascade.