Magnetorotational dynamo chimeras. The missing link to turbulent accretion disk dynamo models?

Abstract

In Keplerian accretion disks, turbulence and magnetic fields may be jointly excited through a subcritical dynamo process involving the magnetorotational instability (MRI). High-resolution simulations exhibit a tendency towards statistical self-organization of MRI dynamo turbulence into large-scale cyclic dynamics. Understanding the physical origin of these structures, and whether they can be sustained and transport angular momentum efficiently in astrophysical conditions, represents a significant theoretical challenge. The discovery of simple periodic nonlinear MRI dynamo solutions has recently proven useful in this respect, and has notably served to highlight the role of turbulent magnetic diffusion in the seeming decay of the dynamics at low magnetic Prandtl number Pm (magnetic diffusivity larger than viscosity), a common regime in accretion disks. The connection between these simple structures and the statistical organization reported in turbulent simulations remained elusive, though. Here, we report the numerical discovery in moderate aspect ratio Keplerian shearing boxes of new periodic, incompressible, three-dimensional nonlinear MRI dynamo solutions with a larger dynamical complexity reminiscent of such simulations. These "chimera" cycles are characterized by multiple MRI-unstable dynamical stages, but their basic physical principles of self-sustainment are nevertheless identical to those of simpler cycles found in azimuthally elongated boxes. In particular, we find that they are not sustained at low Pm either due to subcritical turbulent magnetic diffusion. These solutions offer a new perspective into the transition from laminar to turbulent instability-driven dynamos, and may prove useful to devise improved statistical models of turbulent accretion disk dynamos.

12 pages, 8 figures, submitted to A&A

References in corpus (12)

• MHD simulations of the magnetorotational instability in a shearing box with zero net flux. II. The effect of transport coefficients
• Impact of dimensionless numbers on the efficiency of MRI-induced turbulent transport
• On the Magnetic Prandtl Number Behavior of Accretion Disks
• A self-sustaining nonlinear dynamo process in Keplerian shear flows
• On Self-Sustained Dynamo Cycles in Accretion Discs
• Convection Causes Enhanced Magnetic Turbulence in Accretion Disks in Outburst
• Angular momentum transport and large eddy simulations in magnetorotational turbulence: the small Pm limit
• Statistical simulation of the magnetorotational dynamo
• Subcritical dynamos in shear flows
• Dissipative effects on the sustainment of a magnetorotational dynamo in Keplerian shear flow
• Localized magnetorotational instability and its role in the accretion disc dynamo
• Sustained Turbulence in Differentially Rotating Magnetized Fluids at Low Magnetic Prandtl Number

Cited by in corpus (4)

• Transition to subcritical turbulence in a tokamak plasma
• Nonlinear transverse cascade and sustenance of MRI-turbulence in Keplerian disks with an azimuthal magnetic field
• Gravito-turbulence and dynamo in poorly ionised protostellar discs. I. Zero-net-flux case
• Gravitoturbulent dynamo in global simulations of gaseous disks