  Aims: To determine alpha effect and turbulent magnetic diffusivity for mean
magnetic fields with profiles of different length scale from simulations of
isotropic turbulence, and to relate these results to nonlocal formulations in
which alpha and the turbulent magnetic diffusivity correspond to integral
kernels. Methods: A set of evolution equations for magnetic fields is solved
which gives the response to imposed test fields, that is, mean magnetic fields
with various wavenumbers. Both an imposed fully helical steady flow consisting
of a pattern of screw-like motions (Roberts flow) and time-dependent
statistically steady isotropic turbulence are considered. In the latter case
the aforementioned evolution equations are solved simultaneously with the
momentum and continuity equations. The corresponding results for the
electromotive force are used to calculate alpha and magnetic diffusivity
tensors. Results: For both the Roberts flow under the second--order correlation
approximation and isotropic turbulence alpha and turbulent magnetic diffusivity
are largest at large scales and these values diminish toward smaller scales. In
both cases the alpha effect and turbulent diffusion kernels are well
approximated by exponentials, corresponding to Lorentzian profiles in Fourier
space. For isotropic turbulence the turbulent diffusion kernel is half as wide
as the alpha effect kernel. For the Roberts flow beyond the second--order
correlation approximation the turbulent diffusion kernel becomes negative at
large scales.
