The nonlinear galactic dynamo and magnetic helicity transport

We discuss nonlinear mean-field galactic dynamo models and associate a universal mechanism for saturation of the dynamo with the transport of magnetic helicity. The crucial point for saturation of the dynamo is the existence of a non-zero flux of magnetic helicity. We demonstrate that this saturation mechanism is quite insensitive to the form of this helicity flux. In that sense this is a robust mechanism which limits the growth of the mean magnetic field. Without this flux, the total magnetic helicity is conserved locally and the strength of the saturated mean magnetic field is very small compared to the equipartition strength. The inclusion of a flux of magnetic helicity means that the total magnetic helicity is not conserved locally because the magnetic helicity of small-scale magnetic fluctuations is redistributed by the flux. The equilibrium state is given by a balance between magnetic helicity production and magnetic helicity transport. The equilibrium value of the galactic large-scale magnetic field is given approximately by equipartition between the kinetic energy densities of the interstellar turbulence and the mean magnetic field. We also compare the action of algebraic and dynamic nonlinearities in the galactic dynamo. The algebraic a quenching saturates the dynamo, however a more realistic simultaneous quenching of the a effect and turbulent magnetic diffusion cannot saturate the growth of the mean magnetic field; this can only be achieved by the combined effects of algebraic and dynamic nonlinearities.