Force-free parameter ($\alpha$)

Writing $\nabla \times \mathbf{B}=(4\pi /c)\alpha \mathbf{B}$ defines the force-free parameter $\alpha$. Since the divergence of a curl vanishes and $\nabla \cdot \mathbf{B}=0$, it satisfies $\mathbf{B}\cdot \nabla \alpha =0$.