  Using simulations of slowly rotating stratified turbulence, we show that the
alpha effect responsible for the generation of astrophysical magnetic fields is
proportional to the logarithmic gradient of kinetic energy density rather than
that of momentum, as was previously thought. This result is in agreement with a
new analytic theory developed in this paper for large Reynolds numbers. Thus,
the contribution of density stratification is less important than that of
turbulent velocity. The alpha effect and other turbulent transport coefficients
are determined by means of the test-field method. In addition to forced
turbulence, we also investigate supernova-driven turbulence and stellar
convection. In some cases (intermediate rotation rate for forced turbulence,
convection with intermediate temperature stratification, and supernova-driven
turbulence) we find that the contribution of density stratification might be
even less important than suggested by the analytic theory.
