Astronomy and Astrophysics – Astrophysics – Galaxy Astrophysics
Scientific paper
2012-04-12
Astronomy and Astrophysics
Astrophysics
Galaxy Astrophysics
16 pages, 1 figure
Scientific paper
We study mean field dynamo action in renovating flows with finite and non zero correlation time ($\tau$) in the presence of shear. Previous results obtained when shear was absent are generalized to the case with shear. The question of whether the mean magnetic field can grow in the presence of shear and non helical turbulence, as seen in numerical simulations, is examined. We show in a general manner that, if the motions are strictly non helical, then such mean field dynamo action is not possible. This result is not limited to low (fluid or magnetic) Reynolds numbers nor does it use any closure approximation; it only assumes that the flow renovates itself after each time interval $\tau$. Specifying to a particular form of the renovating flow with helicity, we recover the standard dispersion relation of the $\alpha^2 \Omega$ dynamo, in the small $\tau$ or large wavelength limit. Thus mean fields grow even in the presence of rapidly growing fluctuations, surprisingly, in a manner predicted by the standard quasilinear closure, even though such a closure is not strictly justified. Our work also suggests the possibility of obtaining mean field dynamo growth in the presence of helicity fluctuations, although having a coherent helicity will be more efficient.
Kolekar Sanved
Sridhar Srinath
Subramanian Kandaswamy
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