Generation of large-scale magnetic fields due to fluctuating alpha in shearing systems

Jingade, Naveen and Singh, Nishant K and Sridhar, S (2018) Generation of large-scale magnetic fields due to fluctuating alpha in shearing systems. In: JOURNAL OF PLASMA PHYSICS, 84 (6).

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Abstract

We explore the growth of large-scale magnetic fields in a shear flow, due to helicity fluctuations with a finite correlation time, through a study of the Kraichnan-Moffatt model of zero-mean stochastic fluctuations of the alpha parameter of dynamo theory. We derive a linear integro-differential equation for the evolution of the large-scale magnetic field, using the first-order smoothing approximation and the Galilean invariance of the alpha-statistics. This enables construction of a model that is non-perturbative in the shearing rate S and the alpha-correlation time tau(alpha). After a brief review of the salient features of the exactly solvable white-noise limit, we consider the case of small but non-zero tau(alpha). When the large-scale magnetic field varies slowly, the evolution is governed by a partial differential equation. We present modal solutions and conditions for the exponential growth rate of the large-scale magnetic field, whose drivers are the Kraichnan diffusivity, Moffatt drift, shear and a non-zero correlation time. Of particular interest is dynamo action when the alpha-fluctuations are weak; i.e. when the Kraichnan diffusivity is positive. We show that in the absence of Moffatt drift, shear does not give rise to growing solutions. But shear and Moffatt drift acting together can drive large-scale dynamo action with growth rate gamma proportional to vertical bar S vertical bar.

Item Type:	Journal Article
Publication:	JOURNAL OF PLASMA PHYSICS
Publisher:	CAMBRIDGE UNIV PRESS
Additional Information:	Copyright of this article belongs to CAMBRIDGE UNIV PRESS
Keywords:	astrophysical plasmas
Department/Centre:	Division of Physical & Mathematical Sciences > Physics
Date Deposited:	07 Feb 2019 08:49
Last Modified:	07 Feb 2019 08:49
URI:	http://eprints.iisc.ac.in/id/eprint/61660

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