The Magnetic Prandtl number (Pr_m) is a dimensionless quantity occurring in magnetohydrodynamics which approximates the ratio of momentum diffusivity (viscosity) and magnetic diffusivity. It is defined as:

$${\displaystyle \mathrm {{Pr}_{m}} ={\frac {\mathrm {{Re}_{m}} }{\mathrm {Re} }}={\frac {\nu }{\eta }}={\frac {\text{viscous diffusion rate}}{\text{magnetic diffusion rate}}}}$$

where:

At the base of the Sun's convection zone the Magnetic Prandtl number is approximately 10⁻², and in the interiors of planets and in liquid-metal laboratory dynamos is approximately 10⁻⁵.