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CATEGORIES:DAMTP Astrophysics Seminars
SUMMARY:Transition to MRI dynamo action in Keplerian shear
flows - Mr. Antoine Riols (IRAP\, Toulouse)
DTSTART;TZID=Europe/London:20131028T160000
DTEND;TZID=Europe/London:20131028T170000
UID:TALK48171AThttp://talks.cam.ac.uk
URL:http://talks.cam.ac.uk/talk/index/48171
DESCRIPTION:The magnetorotational (MRI) dynamo is a three-dime
nsional nonlinear magnetohydrodynamic process whos
e most notable feature is its joint ability to exc
ite magnetic fields and MHD turbulence in MRI-unst
able flows such as Keplerian shear flow. This proc
ess may be relevant to explain how angular moment
um can be transported in a variety of accreting en
vironments but is also interesting from a wider dy
namo perspective\, as it offers a very interesting
example of a subcritical instability-driven dynam
o. The detailed mechanisms underlying the transiti
on to MRI dynamo action are not currently understo
od. In particular\, numerical work by Fromang et a
l. (2007) suggests that the MRI dynamo is strongly
affected by dissipative processes and may not be
sustained at low magnetic Prandtl number (Pm = vis
cosity/magnetic diffusivity)\, the most common reg
ime of dissipation in accretion disks. Why and whe
ther this is indeed the case remain to be elucidat
ed. A possible way to address this problem is by s
tudying the bifurcations and transitional nonlinea
r dynamics of the dynamo.\n\nIn this presentation\
, I will present the results of an extensive numer
ical investigation of the role of nonlinear cohere
nt structures\, more specifically MRI dynamo cycle
s\, in the transition. I will show that the emerge
nce of three-dimensional chaotic dynamo action in
the incompressible version of the problem is prima
rily associated with global homoclinic and heteroc
linic bifurcations involving the stable and unstab
le manifolds of such cycles. This result suggests
that dynamo cycles are key actors of the MRI dynam
o transition and raises the hope that the main pro
perties of the transition as a whole may be unders
tood thanks to a meticulous analysis of nonlinear
invariant solutions. I will present several prelim
inary results along these lines and discuss how th
ey may help to understand why the MRI dynamo trans
ition appears to be dependent on the dissipative r
egime considered.
LOCATION:MR14\, Centre for Mathematical Sciences\, Wilberf
orce Road\, Cambridge
CONTACT:Adrian Barker
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