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In Keplerian accretion disks, turbulence and magnetic fields may be jointlyexcited through a subcritical dynamo process involving the magnetorotationalinstability (MRI). High-resolution simulations exhibit a tendency towardsstatistical self-organization of MRI dynamo turbulence into large-scale cyclicdynamics. Understanding the physical origin of these structures, and whetherthey can be sustained and transport angular momentum efficiently inastrophysical conditions, represents a significant theoretical challenge. Thediscovery of simple periodic nonlinear MRI dynamo solutions has recently provenuseful in this respect, and has notably served to highlight the role ofturbulent magnetic diffusion in the seeming decay of the dynamics at lowmagnetic Prandtl number Pm (magnetic diffusivity larger than viscosity), acommon regime in accretion disks. The connection between these simplestructures and the statistical organization reported in turbulent simulationsremained elusive, though. Here, we report the numerical discovery in moderateaspect ratio Keplerian shearing boxes of new periodic, incompressible,three-dimensional nonlinear MRI dynamo solutions with a larger dynamicalcomplexity reminiscent of such simulations. These "chimera" cycles arecharacterized by multiple MRI-unstable dynamical stages, but their basicphysical principles of self-sustainment are nevertheless identical to those ofsimpler cycles found in azimuthally elongated boxes. In particular, we findthat they are not sustained at low Pm either due to subcritical turbulentmagnetic diffusion. These solutions offer a new perspective into the transitionfrom laminar to turbulent instability-driven dynamos, and may prove useful todevise improved statistical models of turbulent accretion disk dynamos.

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