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SUMMARY:Transition to MRI dynamo action in Keplerian shear flows - Mr. Ant
 oine Riols (IRAP\, Toulouse)
DTSTART:20131028T160000Z
DTEND:20131028T170000Z
UID:TALK48171@talks.cam.ac.uk
CONTACT:Adrian Barker
DESCRIPTION:The magnetorotational (MRI) dynamo is a three-dimensional nonl
 inear 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 ho
 w angular momentum can be transported in a variety of accreting environmen
 ts 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 no
 t currently understood. In particular\, numerical work by Fromang et al. (
 2007) suggests that the MRI dynamo is strongly affected by dissipative pro
 cesses and may not be sustained at low magnetic Prandtl number (Pm = visco
 sity/magnetic diffusivity)\, the most common regime of dissipation in accr
 etion disks. Why and whether this is indeed the case remain to be elucidat
 ed. A possible way to address this problem is by studying the bifurcations
  and transitional nonlinear dynamics of the dynamo.\n\nIn this presentatio
 n\, I will present the results of an extensive numerical investigation of 
 the role of nonlinear coherent structures\, more specifically MRI dynamo c
 ycles\, in the transition. I will show that the emergence of three-dimensi
 onal 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 s
 uggests 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 solu
 tions. I will present several preliminary results along these lines and di
 scuss how they may help to understand why the MRI dynamo transition appear
 s to be dependent on the dissipative regime considered.
LOCATION:MR14\,  Centre for Mathematical Sciences\, Wilberforce Road\, Cam
 bridge
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