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Global bifurcations to subcritical turbulent magnetorotational dynamo action in Keplerian shear flow

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Magnetorotational (MRI) dynamo action in Keplerian shear flow is a three-dimensional, nonlinear magnetohydrodynamic
mechanism whose study is relevant to dynamo theory and to the understanding of accretion processes in astrophysics. Transition to
this form of dynamo action is subcritical and shares many of the characteristics of subcritical transition to turbulence in hydrodynamic
shear flows. This suggests that both types of flows become active through similar generic bifurcation mechanisms, which in both cases
have eluded detailed understanding so far. We build on recent work on both types of problems to investigate numerically the bifurcation
mechanisms at work in the MRI dynamo problem. The emergence of three-dimensional chaos and transient magnetohydrodynamic
turbulence in this problem is shown to be primarily associated with global homoclinic and heteroclinic bifurcations involving the stable
and unstable manifolds of nonlinear MRI dynamo cycles born out of saddle node bifurcations. The detailed results strongly suggest
that nonlinear MRI dynamo cycles are key actors of the transition in this system. This opens new perspectives to assess the conditions
of excitation of instability-driven dynamos in Nature and in laboratory experiments.

Author(s):

Francois Rincon    
Institut de Recherche en Astrophysique et Planetologie, Toulouse
France

Antoine Riols    
Institut de Recherche en Astrophysique et Planetologie, Toulouse
France

Carlo Cossu    
Institut de Mecanique des Fluides de Toulouse
France

Geoffroy Lesur    

Pierre-Yves Longaretti    

Gordon Ogilvie    

Johann Herault