= Current helicity
{title2=$H_c=\langle\mathbf b\cdot\nabla\times\mathbf b\rangle$}
Current helicity correlates a <magnetic field> with its <curl>, which is proportional to <electric current density>. It differs from <magnetic helicity>, which uses a <magnetic vector potential>. For a solenoidal monochromatic field of wavenumber $k$ in a periodic domain, choose $\mathbf a_b=\nabla\times\mathbf b/k^2$. Then $\nabla\times\mathbf a_b=\mathbf b$ and $H_c=k^2\langle\mathbf a_b\cdot\mathbf b\rangle$.
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