= Turbulent magnetic diffusivity
{title2=$\eta_t$}
Turbulent magnetic diffusivity is an effective transport coefficient produced by correlated small-scale <velocity> and <magnetic field>. It supplements microscopic <magnetic diffusivity> in a <mean-field dynamo> description. Its identification requires a specified mean-field geometry and averaging procedure; in a planar flux problem, the correlation $\overline{wa'}=-\eta_tB_0f(z)$ describes transport relative to a prescribed uniform field.
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