Magnetic Reynolds number
= Magnetic Reynolds number
{title2=$\operatorname{Rm}=UL/\eta_m$}
The magnetic Reynolds number compares field advection with resistive <magnetic diffusion>. With constant <electrical conductivity> and vacuum permeability, magnetic diffusivity is $\eta_m=1/(\mu_0\sigma)$ and $\operatorname{Rm}=UL/\eta_m$. The ideal induction approximation applies when this ratio is large on the scales of interest.