Alfvén speed ($v_A$)

The Alfvén speed associated with magnetic-field magnitude $B$ and density $\rho$ is $v_A=B/\sqrt{{\mu }_0\rho }$.

An Alfvén number is the ratio of a flow speed to the corresponding Alfvén speed. For a poloidal flow parallel to a poloidal magnetic field, $A^2={\mu }_0\rho u_p^2/B_p^2$.