Origin of nonlinearity and plausible turbulence by hydromagnetic transient growth in accretion disks: Faster growth rate than magnetorotational instability

TL;DR

This paper demonstrates that in accretion disks, hydromagnetic transient growth causes nonlinearity and potential turbulence faster than magnetorotational instability, especially at high Reynolds numbers typical of astrophysical environments.

Contribution

It shows that transient growth can induce nonlinearity more rapidly than MRI in realistic accretion disk conditions, challenging the traditional emphasis on MRI.

Findings

Transient growth leads to faster nonlinearity than MRI at high Reynolds numbers.

MRI dominates only at low Reynolds numbers and weak magnetic fields.

Results question the overall effectiveness of MRI in astrophysical accretion disks.

Abstract

We investigate the evolution of hydromagnetic perturbations in a small section of accretion disks. It is known that molecular viscosity is negligible in accretion disks. Hence, it has been argued that a mechanism, known as Magnetorotational Instability (MRI), is responsible for transporting matter in the presence of weak magnetic field. However, there are some shortcomings, which question effectiveness of MRI. Now the question arises, whether other hydromagnetic effects, e.g. transient growth (TG), can play important role to bring nonlinearity in the system, even at weak magnetic fields. Otherwise, whether MRI or TG, which is primarily responsible to reveal nonlinearity to make the flow turbulent? Our results prove explicitly that the flows with high Reynolds number (Re), which is the case of realistic astrophysical accretion disks, exhibit nonlinearity by TG of perturbation modes faster than that by modes producing MRI. For a fixed wave vector, MRI dominates over transient effects, only at low Re, lower than its value expected to be in astrophysical accretion disks, and low magnetic fields. This seriously questions (overall) persuasiveness of MRI in astrophysical accretion disks.