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SUMMARY:Mean field responses in disordered systems: an example from nonlin
 ear MHD - David Hughes (Leeds)
DTSTART:20251117T160000Z
DTEND:20251117T170000Z
UID:TALK237610@talks.cam.ac.uk
CONTACT:Loren E. Held
DESCRIPTION:Understanding the generation of large-scale magnetic fields an
 d flows in MHD turbulence remains one of the most challenging problems in 
 astrophysical fluid dynamics. The well-studied kinematic theory ignores th
 e possibility of small-scale dynamo action\, which is prevalent at high ma
 gnetic Reynolds number. Here\, we thus examine the possibility of large-sc
 ale field (and flow) arising from a fully MHD basic state — as may arise
  from the saturation of a small-scale dynamo. Under this new theory\, four
  mean field tensors now come into play. In certain cases\, such as "short-
 sudden" turbulence\, these tensors may be calculated explicitly. More gene
 rally\, the (nonlinear) mean field theory works well for mildly turbulent 
 flows\, but can fail in strongly disordered systems. This leads us to addr
 ess the more general dynamical systems question of whether a linear respon
 se can be detected following a  small (linear) perturbation to a chaotic 
 system.
LOCATION:MR14 DAMTP and online
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