Alpha effect and diffusivity in helical turbulence with shear

Aims. We study the dependence of turbulent transport coefficients, such as α effect and turbulent magnetic diffusivity, on shear and magnetic Reynolds number in the presence of helical forcing. Methods. We use three-dimensional direct numerical simulations with periodic boundary conditions and measure turbulent transport coefficients using the kinematic test field method. In all cases the magnetic Prandtl number is unity. Results. We obtain numerical evidence for quenching of the diagonal components of α tensor with increasing shear. We also calculate the diffusivity tensor and find that the diagonal components of the turbulent diffusivity tensor increases with shear. We further study the behaviour of the off-diagonal components of the turbulent diffusivity tensor and find evidence of a sign change at high magnetic Reynolds number and shear in a way that may be suggestive of a shear–current effect.