= First-order smoothing approximation
= Second-order correlation approximation
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= FOSA
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First-order smoothing neglects the fluctuating nonlinear velocity-magnetic-field product in the fluctuation <resistive induction equation>, but retains $\langle\mathbf u\times\mathbf b\rangle$ in the <mean-field electromotive force>. For a constant test field it gives $\partial_t\mathbf b-\eta\nabla^2\mathbf b=(\mathbf B_0\cdot\nabla)\mathbf u$. Its validity requires the omitted terms to be small compared with the retained forcing and diffusion terms.
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