Nishant K. Singh

Large-scale dynamo action due to α fluctuations in a linear shear flow

We present a model of large-scale dynamo action in a shear flow that has stochastic, zero-mean fluctuations of the a parameter. This is based on a minimal extension of the Kraichnan Moffatt model, to include a background linear shear and Galilean-invariant alpha-statistics. Using the firstorder smoothing approximation we derive a linear integro-differential equation for the largescale magnetic field, which is non-perturbative in the shearing rate S, and the alpha-correlation time r. The white-noise case, tau(alpha) = 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 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 r, gives rise to new physical scales, and dynamo action is completely different from the white-noise case; e.g. even weak a 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.

HIGH-WAVENUMBER SOLAR f-MODE STRENGTHENING PRIOR TO ACTIVE REGION FORMATION

We report a systematic strengthening of the local solar surface mode, i.e. the

NUMERICAL STUDIES OF DYNAMO ACTION IN A TURBULENT SHEAR FLOW. I.

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Two-scale Analysis of Solar Magnetic Helicity

-mode, 1-2 days prior to the emergence of an active region (AR) in the same (corotating) location while no indication can yet be seen in the magnetograms. Our study is motivated by earlier numerical findings of Singh et al. (2014) which showed that, in the presence of a nonuniform magnetic field that is concentrated a few scale heights below the surface, the

Enhancement of Small-scale Turbulent Dynamo by Large-scale Shear

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Bihelical Spectrum of Solar Magnetic Helicity and Its Evolution

-mode fans out in the diagnostic

A new constraint on mean-field galactic dynamo theory

k\omega

Properties of p and f modes in hydromagnetic turbulence

diagram at high wavenumbers. Here we explore this possibility using data from the Helioseismic and Magnetic Imager on board the {\em Solar Dynamics observatory}, and show for four ARs 11130, 11158, 11768, and 12051, that at large latitudinal wavenumbers (corresponding to horizontal scales of around 3000 km), the

Non-linear galactic dynamos and the magnetic Rädler effect

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Plane shearing waves of arbitrary form: Exact solutions of the Navier-Stokes equations

-mode displays strengthening about two days prior to AR formation and thus provides a new precursor for AR formation. The idea that the