English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Tools annual

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Presents the access of premium content as premium feature

Premium Content

Global: Zendy Plus annual

Global: Zendy Free Plan

We present a theory of large-scale dynamo action in a turbulent flow that hasstochastic, zero-mean fluctuations of the $\alpha$ parameter. Particularlyinteresting is the possibility of the growth of the mean magnetic field due toMoffatt drift, which is expected to be finite in a statistically anisotropicturbulence. We extend the Kraichnan-Moffatt model to explore effects of finitememory of $\alpha$ fluctuations, in a spirit similar to that of Sridhar & Singh(2014), hereafter SS14. Using the first-order smoothing approximation, wederive a linear integro-differential equation governing the dynamics of thelarge-scale magnetic field, which is non-perturbative in the$\alpha$-correlation time $\tau_{\alpha}$. We recover earlier results in theexactly solvable white-noise (WN) limit where the Moffatt drift does notcontribute to the dynamo growth/decay. To study finite memory effects, wereduce the integro-differential equation to a partial differential equation byassuming that the $\tau_{\alpha}$ be small but nonzero and the large-scalemagnetic field is slowly varying. We derive the dispersion relation and provideexplicit expression for the growth rate as a function of four independentparameters. When $\tau_{\alpha}\neq 0$, we find that: (i) in the absence of theMoffatt drift, but with finite Kraichnan diffusivity, only strong$\alpha$-fluctuations can enable a mean-field dynamo; (ii) in the general casewhen also the Moffatt drift is nonzero, both, weak or strong $\alpha$fluctuations, can lead to a large-scale dynamo; and (iii) there always exists awavenumber ($k$) cutoff at some large $k$ beyond which the growth rate turnsnegative. Thus we show that a finite Moffatt drift can always facilitatelarge-scale dynamo action if sufficiently strong, even in case of weak $\alpha$fluctuations, and the maximum growth occurs at intermediate wavenumbers.

Moffatt drift driven large scale dynamo due to $\alpha$ fluctuations with nonzero correlation times