  We investigate analytically the amplification of a weak magnetic field in a
homogeneous and isotropic turbulent flow lacking reflectional symmetry (helical
turbulence). We propose that the spectral distributions of magnetic energy and
magnetic helicity can be found as eigenmodes of a self-adjoint,
Schr\"odinger-type system of evolution equations. We argue that large-scale and
small-scale magnetic fluctuations cannot be effectively separated, and that the
conventional alpha-model is, in general, not an adequate description of the
large-scale dynamo mechanism. As a consequence, the correct numerical modeling
of such processes should resolve magnetic fluctuations down to the very small,
resistive scales.
