 Conductivity-gradient term in magnetic induction

For scalar electrical conductivity $\sigma (\mathbf{x})$ and constant permeability ${\mu }_0$, the moving-fluid Ohm law and the quasistatic Ampere-Maxwell law give $\mathbf{E}=-\mathbf{u}\times \mathbf{B}+\eta \nabla \times \mathbf{B}$, where $\eta =1/({\mu }_0\sigma )$ is the magnetic diffusivity. Taking minus the curl and using Faraday's law of induction gives ${\partial }_t\mathbf{B}=\nabla \times (\mathbf{u}\times \mathbf{B})-\nabla \times (\eta \nabla \times \mathbf{B})$. Expanding this expression and imposing $\nabla \cdot \mathbf{B}=0$ yields the title equation. The familiar constant-diffusivity resistive induction equation omits the last term because $\nabla \eta =0$; nonrelativistic motion alone does not establish that simplification.