Abstract:
We present a model for a global axisymmetric turbulent dynamo operating in a galaxy with a corona that treats the
parameters of turbulence driven by supernovae and by magneto-rotational instability under a common formalism.
The nonlinear quenching of the dynamo is alleviated by the inclusion of small-scale advective and diffusive
magnetic helicity
fl
uxes, which allow the gauge-invariant magnetic helicity to be transferred outside the disk and
consequently to build up a corona during the course of dynamo action. The time-dependent dynamo equations are
expressed in a separable form and solved through an eigenvector expansion constructed using the steady-state
solutions of the dynamo equation. The parametric evolution of the dynamo solution allows us to estimate the
fi
nal
structure of the global magnetic
fi
eld and the saturated value of the turbulence parameter
α
m
, even before solving
the dynamical equations for evolution of magnetic
fi
elds in the disk and the corona, along with
α
-quenching. We
then solve these equations simultaneously to study the saturation of the large-scale magnetic
fi
eld, its dependence
on the small-scale magnetic helicity
fl
uxes, and the corresponding evolution of the force-free
fi
eld in the corona.
The quadrupolar large-scale magnetic
fi
eld in the disk is found to reach equipartition strength within a timescale of
1 Gyr. The large-scale magnetic
fi
eld in the corona obtained is much weaker than the
fi
eld inside the disk and has
only a weak impact on the dynamo operation.
