Title: The dynamo effect in decaying helical turbulence,
Abstract: We show that in decaying hydromagnetic turbulence with initial kinetic
helicity, a weak magnetic field eventually becomes fully helical. The sign of
magnetic helicity is opposite to that of the kinetic helicity - regardless of
whether or not the initial magnetic field was helical. The magnetic field
undergoes inverse cascading with the magnetic energy decaying approximately
like t^{-1/2}. This is even slower than in the fully helical case, where it
decays like t^{-2/3}. In this parameter range, the product of magnetic energy
and correlation length raised to a certain power slightly larger than unity, is
approximately constant. This scaling of magnetic energy persists over long time
scales. At very late times and for domain sizes large enough to accommodate the
growing spatial scales, we expect a cross-over to the t^{-2/3} decay law that
is commonly observed for fully helical magnetic fields. Regardless of the
presence or absence of initial kinetic helicity, the magnetic field experiences
exponential growth during the first few turnover times, which is suggestive of
small-scale dynamo action. Our results have applications to a wide range of
experimental dynamos and astrophysical time-dependent plasmas, including
primordial turbulence in the early universe.