Possible origin of viscosity in the Keplerian accretion disks due to secondary perturbation: Turbulent transport without magnetic field

TL;DR

This paper proposes that secondary perturbations and elliptical instability can generate turbulence and effective viscosity in Keplerian accretion disks without magnetic fields, explaining angular momentum transport.

Contribution

It introduces a hydrodynamic mechanism involving secondary disturbances and elliptical instability as a source of turbulence in magnetically neutral accretion disks.

Findings

Secondary perturbations can trigger elliptical instability.

Turbulent viscosity range is estimated between 0.0001 and 0.1.

Hydrodynamic turbulence can explain accretion disk transport without magnetic fields.

Abstract

The origin of hydrodynamic turbulence in rotating shear flow is a long standing puzzle. Resolving it is especially important in astrophysics when the flow angular momentum profile is Keplerian which forms an accretion disk having negligible molecular viscosity. Hence, any viscosity in such systems must be due to turbulence, arguably governed by magnetorotational instability especially when temperature T >~ 10^5. However, such disks around quiescent cataclysmic variables, protoplanetary and star-forming disks, the outer regions of disks in active galactic nuclei are practically neutral in charge because of their low temperature, and thus expected not to be coupled with the magnetic field appropriately to generate any transport due to the magnetorotational instability. This flow is similar to plane Couette flow including the Coriolis force, at least locally. What drives their turbulence and then transport, when such flows do not exhibit any unstable mode under linear hydrodynamic perturbation? We demonstrate that the threedimensional secondary disturbance to the primarily perturbed flow triggering elliptical instability may generate significant turbulent viscosity ranging 0.0001 <~ \nu_t <~ 0.1 to explain transport in accretion flows.