Growth of a localized seed magnetic field in a turbulent medium
  Turbulence dynamo deals with amplification of a seed magnetic field in a
turbulent medium and has been studied mostly for uniform or spatially
homogeneous seed magnetic fields. However, some astrophysical processes (e.g.
jets from active galaxies, galactic winds, or ram-pressure stripping in galaxy
clusters) can provide localized seed magnetic fields. In this paper, we
numerically study amplification of localized seed magnetic fields in a
turbulent medium. Throughout the paper, we assume that driving scale of
turbulence is comparable to the size of the system. Our findings are as
follows. First, turbulence can amplify a localized seed magnetic field very
efficiently. The growth rate of magnetic energy density is as high as that for
a uniform seed magnetic field. This result implies that a magnetic field
ejected from an astrophysical object can be a viable source of magnetic field
in a cluster. Second, the localized seed magnetic field disperses and fills the
whole system very fast. If turbulence in a system (e.g. a galaxy cluster or a
filament) is driven at large scales, we expect that it takes a few large-eddy
turnover times for magnetic field to fill the whole system. Third, growth and
turbulence diffusion of a localized seed magnetic field are also fast in high
magnetic Prandtl number turbulence. Fourth, even in decaying turbulence, a
localized seed magnetic field can ultimately fill the whole system. Although
the dispersal rate of magnetic field is not fast in purely decaying turbulence,
it can be enhanced by an additional forcing.
