Magnetorotational dynamo chimeras

The missing link to turbulent accretion disk dynamo models?^⋆

¹ Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Centre for Mathematical Sciences, Wilberforce Road, Cambridge CB3 0WA, UK
e-mail: ar764@cam.ac.uk
² Université de Toulouse, UPS-OMP, IRAP Toulouse, 31400 Toulouse, France
e-mail: francois.rincon@irap.omp.eu
³ CNRS, IRAP, 14 avenue Édouard Belin, 31400 Toulouse, France
⁴ Institut de Mécanique des Fluides de Toulouse (IMFT), CNRS – Université de Toulouse, Allée du Professeur Camille Soula, 31400 Toulouse, France
⁵ CNRS, IPAG, 38000 Grenoble, France
⁶ Univ. Grenoble Alpes, IPAG, 38000 Grenoble, France

Received: 11 July 2016
Accepted: 21 October 2016

Abstract

In Keplerian accretion disks, turbulence and magnetic fields may be jointly excited through a subcritical dynamo mechanisminvolving magnetorotational instability (MRI). This dynamo may notably contribute to explaining the time-variability of various accreting systems, as high-resolution simulations of MRI dynamo turbulence exhibit statistical self-organization into large-scale cyclic dynamics. However, understanding the physics underlying these statistical states and assessing their exact astrophysical relevance is theoretically challenging. The study of simple periodic nonlinear MRI dynamo solutions has recently proven useful in this respect, and has highlighted the role of turbulent magnetic diffusion in the seeming impossibility of a dynamo at low magnetic Prandtl number (Pm), a common regime in disks. Arguably though, these simple laminar structures may not be fully representative of the complex, statistically self-organized states expected in astrophysical regimes. Here, we aim at closing this seeming discrepancy by reporting the numerical discovery of exactly periodic, yet semi-statistical “chimeral MRI dynamo states” which are the organized outcome of a succession of MRI-unstable, non-axisymmetric dynamical stages of different forms and amplitudes. Interestingly, these states, while reminiscent of the statistical complexity of turbulent simulations, involve the same physical principles as simpler laminar cycles, and their analysis further confirms the theory that subcritical turbulent magnetic diffusion impedes the sustainment of an MRI dynamo at low Pm. Overall, chimera dynamo cycles therefore offer an unprecedented dual physical and statistical perspective on dynamos in rotating shear flows, which may prove useful in devising more accurate, yet intuitive mean-field models of time-dependent turbulent disk dynamos.