Turbulent magnetic Prandtl number and magnetic diffusivity quenching from simulations

Forced turbulence simulations are used to determine the turbulent kinematic viscosity, v(t), from the decay rate of a large scale velocity field. Likewise, the turbulent magnetic diffusivity, ..., is determined from the decay of a large scale magnetic field. In the kinematic regime, when the field is weak, the turbulent magnetic Prandtl number, ..., is about unity. When the field is nonhelical, ...is quenched when magnetic and kinetic energies become comparable. For helical fields the quenching is stronger and can be described by a dynamical quenching formula.