= Monochromatic coupled magnetic and velocity response
{title2=$D=\nu\eta k^4+(\mathbf B\cdot\mathbf q)^2/(\mu_0\rho)$}
For a steady forced <Fourier mode> with $|\mathbf q|=k$, coupled viscous and magnetic response gives $\widehat{\mathbf u}=[\eta k^2\widehat{\mathbf f}+i(\mathbf B\cdot\mathbf q)\widehat{\mathbf g}/(\mu_0\rho)]/D$ and $\widehat{\mathbf b}=[\nu k^2\widehat{\mathbf g}+i(\mathbf B\cdot\mathbf q)\widehat{\mathbf f}]/D$. Their <mean-field electromotive force> contains both forcing helicities and a possible weighted cross-forcing correlation. Jointly isotropic forcing removes the latter; its vanishing at zero mean field alone is insufficient for anisotropic forcing.
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