The Magnetic Prandtl number (Prm) is a dimensionless quantity occurring in magnetohydrodynamics which approximates the ratio of momentum diffusivity (viscosity) and magnetic diffusivity. It is defined as:
where:
At the base of the Sun's convection zone the Magnetic Prandtl number is approximately 10, and in the interiors of planets and in liquid-metal laboratory dynamos is approximately 10.
![\mathrm{Pr}_\mathrm{m} = \frac{\mathrm{Re_m}}{\mathrm{Re}} = \frac{\nu}{\eta} = \frac{\mbox{viscous diffusion rate}}{\mbox{magnetic diffusion rate}}](/Images/godic/202501/25/596995d670c716b00474cd17db9f25c30005.png")