The Magnetic Prandtl number (Prm) is a dimensionless quantity occurring in magnetohydrodynamics which approximates the ratio of momentum diffusivity (viscosity) and magnetic diffusivity.
[1] It is defined as: where: At the base of the Sun's convection zone the Magnetic Prandtl number is approximately 10−2,[2] and in the interiors of planets and in liquid-metal laboratory dynamos is approximately 10−5.