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

Jingade, Naveen; Singh, Nishant K.; Sridhar, S.

doi:10.1017/s0022377818001174

Cited by 7 publications

(20 citation statements)

References 39 publications

(122 reference statements)

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“…We then studied more general case with finite shear and m 2, i.e., when the helicity correlation times are longer than that of the velocity field, which is taken to be on the order of the eddy turnover time T . This work thus further generalizes the work of Jingade et al (2018) to include the non-local effects with memory and, in a sense, role of tensorial α−fluctuations, by self-consistently incorporating the effect of shear on the stochastic velocity and helicity fields.…”

Section: Discussionmentioning

confidence: 70%

“…Most of these studies found the possibility of growth in the second moment of the mean magnetic field, i.e., mean magnetic energy. First moment could grow only as a result of negative turbulent diffusion caused by strong α fluctuations (Kraichnan 1976;Mitra & Brandenburg 2012;Jingade et al 2018). Net negative diffusion is not observed in the simulations of Brandenburg et al (2008) in the range of parameters explored where shear dynamo was found to operate.…”

Section: Introductionmentioning

confidence: 83%

“…fect (Vishniac & Brandenburg 1997;Brandenburg et al 2008;Heinemann et al 2011;Mitra & Brandenburg 2012;Sridhar & Singh 2014;Jingade et al 2018;Käpylä et al 2020). Our interest in this and other related earlier works has been on the kinematic problem when initial seed magnetic fields are weak.…”

Section: Introductionmentioning

confidence: 99%

“…In a double-averaging scheme (see also Moffatt 1983, for renormalisation group technique) as proposed in Kraichnan (1976), the fluctuations in helicity lead to α-fluctuations, which cause a decrement in the turbulent diffusion. Sufficiently strong α fluctuations could lead to LSD by the process of negative turbulent diffusion; see Jingade et al (2018) for a generalized model that also includes memory effects as well as the spatial fluctuations in α leading to the Moffatt drift (Moffatt 1978). This has found applications in the studies of objects such as, the Sun (Silant'ev 2000), accretion disks (Vishniac & Brandenburg 1997), and galaxies (Sokolov 1997).…”

Section: Introductionmentioning

confidence: 99%

“…However, a new idea emerged in these studies, in terms of the Moffatt drift, which would be finite in case of statistically anisotropic α fluctuations. With finite memory, this alone could drive an LSD (Singh 2016), and the role of shear is to further aid the dynamo action (Sridhar & Singh 2014;Jingade et al 2018). Nevertheless, the challenge to explain the growth of mean-magnetic field, the first moment, in a simpler setting with purely temporal α fluctuations, remained.…”

Section: Introductionmentioning

confidence: 99%

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Mean field dynamo action in shear flows. I: fixed kinetic helicity

Jingade

Singh

2020

Monthly Notices of the Royal Astronomical Society

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We study mean field dynamo action in a background linear shear flow by employing pulsed renewing flows with fixed kinetic helicity and non-zero correlation time (τ). We use plane shearing waves in terms of time-dependent exact solutions to the Navier–Stokes equation as derived by Singh & Sridhar (2017). This allows us to self-consistently include the anisotropic effects of shear on the stochastic flow. We determine the average response tensor governing the evolution of mean magnetic field, and study the properties of its eigenvalues that yield the growth rate (γ) and the cycle period (Pcyc) of the mean magnetic field. Both, γ and the wavenumber corresponding to the fastest growing axisymmetric mode vary non-monotonically with shear rate S when τ is comparable to the eddy turnover time T, in which case, we also find quenching of dynamo when shear becomes too strong. When $\tau /T\sim {\cal O}(1)$, the cycle period (Pcyc) of growing dynamo wave scales with shear as Pcyc ∝ |S|−1 at small shear, and it becomes nearly independent of shear as shear becomes too strong. This asymptotic behaviour at weak and strong shear has implications for magnetic activity cycles of stars in recent observations. Our study thus essentially generalizes the standard αΩ (or α2Ω) dynamo as also the α effect is affected by shear and the modelled random flow has a finite memory.

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Section: Discussionmentioning

confidence: 70%

Section: Introductionmentioning

confidence: 83%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Mean field dynamo action in shear flows. I: fixed kinetic helicity

Jingade

Singh

2020

Monthly Notices of the Royal Astronomical Society

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Correlation times of velocity and kinetic helicity fluctuations in non-helical hydrodynamic turbulence

Zhou,

Jingade

2024

J. Fluid Mech.

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Non-helical turbulence within a linear shear flow has demonstrated efficient amplification of large-scale magnetic fields in numerical simulations, but its precise mechanism remains elusive. The incoherent $\alpha$ mechanism proposes that a zero-mean fluctuating transport coefficient $\alpha$ (linked to kinetic helicity) in the shear flow is a candidate driver. Previous renovating-flow models have proposed that the correlation time of helicity fluctuations must be sufficiently extended to overcome turbulent magnetic diffusivity, yet only empirical validation of this concept has been obtained. In this study, we conduct direct numerical simulations of weakly compressible non-helical hydrodynamic turbulence. We scrutinize the correlation times of velocity and kinetic helicity fluctuations in distinct flow configurations, including rotation, shearing and Keplerian flows, as well as the shearing burgulence counterpart. Our findings indicate that rotation contributes to a prolonged correlation time of helicity compared with velocity, particularly notable in auto-correlations of both volume-averaged quantities and individual Fourier modes due to the formation of large-scale vortices. In contrast, moderate shear strength does not exhibit significant scale separation, with shear flows elongating vortices in the shear direction. Shearing burgulence, characterized by shorter helicity correlation times, appears less conducive to hosting the incoherent $\alpha$ effect. Notably, at modest shear rates, only Keplerian flows exhibit sufficiently coherent helicity fluctuations, in contrast to shearing flows. However, the relative strength of helicity fluctuations compared with turbulent diffusivity is significantly lower, raising doubts about the viability of the incoherent $\alpha$ effect as a potential dynamo driver in the subsonic flows examined in this study.

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MHD turbulence: a biased review

Schekochihin

2022

J. Plasma Phys.

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This review of scaling theories of magnetohydrodynamic (MHD) turbulence aims to put the developments of the last few years in the context of the canonical time line (from Kolmogorov to Iroshnikov–Kraichnan to Goldreich–Sridhar to Boldyrev). It is argued that Beresnyak's (valid) objection that Boldyrev's alignment theory, at least in its original form, violates the Reduced-MHD rescaling symmetry can be reconciled with alignment if the latter is understood as an intermittency effect. Boldyrev's scalings, a version of which is recovered in this interpretation, and the concept of dynamic alignment (equivalently, local 3D anisotropy) are thus an example of a physical theory of intermittency in a turbulent system. The emergence of aligned structures naturally brings into play reconnection physics and thus the theory of MHD turbulence becomes intertwined with the physics of tearing, current-sheet disruption and plasmoid formation. Recent work on these subjects by Loureiro, Mallet et al. is reviewed and it is argued that we may, as a result, finally have a reasonably complete picture of the MHD turbulent cascade (forced, balanced, and in the presence of a strong mean field) all the way to the dissipation scale. This picture appears to reconcile Beresnyak's advocacy of the Kolmogorov scaling of the dissipation cutoff (as $\mathrm {Re}^{3/4}$ ) with Boldyrev's aligned cascade. It turns out also that these ideas open the door to some progress in understanding MHD turbulence without a mean field – MHD dynamo – whose saturated state is argued to be controlled by reconnection and to contain, at small scales, a tearing-mediated cascade similar to its strong-mean-field counterpart (this is a new result). On the margins of this core narrative, standard weak-MHD-turbulence theory is argued to require some adjustment – and a new scheme for such an adjustment is proposed – to take account of the determining part that a spontaneously emergent 2D condensate plays in mediating the Alfvén-wave cascade from a weakly interacting state to a strongly turbulent (critically balanced) one. This completes the picture of the MHD cascade at large scales. A number of outstanding issues are surveyed: imbalanced turbulence (for which a new, tentative theory is proposed), residual energy, MHD turbulence at subviscous scales, and decaying MHD turbulence (where there has been dramatic progress recently, and reconnection again turned out to feature prominently). Finally, it is argued that the natural direction of research is now away from the fluid MHD theory and into kinetic territory – and then, possibly, back again. The review lays no claim to objectivity or completeness, focusing on topics and views that the author finds most appealing at the present moment.

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

Cited by 7 publications

References 39 publications

Mean field dynamo action in shear flows. I: fixed kinetic helicity

Mean field dynamo action in shear flows. I: fixed kinetic helicity

Correlation times of velocity and kinetic helicity fluctuations in non-helical hydrodynamic turbulence

MHD turbulence: a biased review

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