2008 (IPP)

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Author: search.filters.author.Sur, Sharanya

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    Kinematic alpha effect in isotropic turbulence simulations
    (2008-01) Sur, Sharanya; Brandenburg, Axel; Subramanian, Kandaswamy
    Using numerical simulations at moderate magnetic Reynolds numbers up to 220 it is shown that in the kinematic regime, isotropic helical turbulence leads to an alpha effect and a turbulent diffusivitywhose values are independent of the magnetic Reynolds number,Rm, provided Rm exceeds unity. These turbulent coefficients are also consistent with expectations from the first order smoothing approximation. For small values of Rm, alpha and turbulent diffusivity are proportional to Rm. Over finite time intervals meaningful values of alpha and turbulent diffusivity can be obtained even when there is small-scale dynamo action that produces strong magnetic fluctuations. This suggests that small-scale dynamo-generated fields do not make a correlated contribution to the mean electromotive force.
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    Galactic dynamo action in presence of stochastic alpha and shear
    (2008-10) Sur, Sharanya; Subramanian, Kandaswamy
    Using a one-dimensional αω-dynamo model appropriate to galaxies, we study the possibility of dynamo action driven by a stochastic alpha effect and shear. To determine the field evolution, one needs to examine a large number of different realizations of the stochastic component of α. The net growth or decay of the field depends not only on the dynamo parameters but also on the particular realization, the correlation time of the stochastic α compared to turbulent diffusion timescale and the time over which the system is evolved. For dynamos where both a coherent and fluctuating α are present, the stochasticity of α can help alleviate catastrophic dynamo quenching, even in the absence of helicity fluxes. One can obtain final field strengths up to a fraction ∼ 0.01 of the equipartition field Beq for dynamo numbers |D| ∼ 40, while fields comparable to Beq require much larger degree of α fluctuations or shear. This type of dynamomay be particularly useful for amplifying fields in the central regions of disk galaxies.