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Transition to MRI dynamo action in Keplerian shear flows

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The magnetorotational (MRI) dynamo is a three-dimensional nonlinear magnetohydrodynamic process whose most notable feature is its joint ability to excite magnetic fields and MHD turbulence in MRI -unstable flows such as Keplerian shear flow. This process may be relevant to explain how angular momentum can be transported in a variety of accreting environments but is also interesting from a wider dynamo perspective, as it offers a very interesting example of a subcritical instability-driven dynamo. The detailed mechanisms underlying the transition to MRI dynamo action are not currently understood. In particular, numerical work by Fromang et al. (2007) suggests that the MRI dynamo is strongly affected by dissipative processes and may not be sustained at low magnetic Prandtl number (Pm = viscosity/magnetic diffusivity), the most common regime of dissipation in accretion disks. Why and whether this is indeed the case remain to be elucidated. A possible way to address this problem is by studying the bifurcations and transitional nonlinear dynamics of the dynamo.

In this presentation, I will present the results of an extensive numerical investigation of the role of nonlinear coherent structures, more specifically MRI dynamo cycles, in the transition. I will show that the emergence of three-dimensional chaotic dynamo action in the incompressible version of the problem is primarily associated with global homoclinic and heteroclinic bifurcations involving the stable and unstable manifolds of such cycles. This result suggests that dynamo cycles are key actors of the MRI dynamo transition and raises the hope that the main properties of the transition as a whole may be understood thanks to a meticulous analysis of nonlinear invariant solutions. I will present several preliminary results along these lines and discuss how they may help to understand why the MRI dynamo transition appears to be dependent on the dissipative regime considered.

This talk is part of the DAMTP Astrophysics Seminar series.

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