Emergence of nonlinearity and plausible turbulence in accretion disks via hydromagnetic transient growth faster than magnetorotational instability

TL;DR

This paper demonstrates that transient growth of hydromagnetic perturbations can induce nonlinearity and turbulence in accretion disks more rapidly than magnetorotational instability, especially at high Reynolds numbers typical of astrophysical disks.

Contribution

It shows that transient growth effects are more effective than MRI in triggering nonlinearity in accretion disks at realistic high Reynolds numbers.

Findings

Transient growth induces nonlinearity faster than MRI at high Re.

MRI dominates at low Re and weak magnetic fields, contrary to astrophysical expectations.

Results challenge 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 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 an 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 best TG of perturbation modes faster than that by best modes producing MRI. For a fixed wavevector, 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.