Title:	Large-scale dynamo action due to α alpha fluctuations in a linear shear flow
Authors:	Sridhar, S. Singh, Nishant K.
Keywords:	MHD turbulence magnetic fields
Issue Date:	4-Dec-2014
Publisher:	Oxford University Press for The Royal Astronomical Society
Citation:	Monthly Notices of the Royal Astronomical Society, 2014, Vol. 445, p3770-3787
Abstract:	We present a model of large-scale dynamo action in a shear flow that has stochastic, zero-mean fluctuations of the α parameter. This is based on a minimal extension of the Kraichnan–Moffatt model, to include a background linear shear and Galilean-invariant α-statistics. Using the first-order smoothing approximation we derive a linear integro-differential equation for the large-scale magnetic field, which is non-perturbative in the shearing rate S , and the α-correlation time τα . The white-noise case, τα = 0 , is solved exactly, and it is concluded that the necessary condition for dynamo action is identical to the Kraichnan–Moffatt model without shear; this is because white-noise does not allow for memory effects, whereas shear needs time to act. To explore memory effects we reduce the integro-differential equation to a partial differential equation, valid for slowly varying fields when τα is small but non-zero. Seeking exponential modal solutions, we solve the modal dispersion relation and obtain an explicit expression for the growth rate as a function of the six independent parameters of the problem. A non-zero τα gives rise to new physical scales, and dynamo action is completely different from the white-noise case; e.g. even weak α fluctuations can give rise to a dynamo. We argue that, at any wavenumber, both Moffatt drift and Shear always contribute to increasing the growth rate. Two examples are presented: (a) a Moffatt drift dynamo in the absence of shear and (b) a Shear dynamo in the absence of Moffatt drift.
Description:	Open Access
URI:	http://hdl.handle.net/2289/6170
ISSN:	0035-8711 1365-2966 - (online)
Alternative Location:	http://adsabs.harvard.edu/abs/2014MNRAS.445.3770S http://arxiv.org/abs/1306.2495 http://dx.doi.org/10.1093/mnras/stu1981
Copyright:	2014 The authors & the Royal Astronomical Society
Appears in Collections:	Research Papers (A&A)

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