The excitation and further sustenance of large-scale magnetic fields in rotating astrophysical systems, including planets, stars, and galaxies, is generally thought to involve a fluid magnetic dynamo effect driven by helical magnetohydrodynamic turbulence. While this scenario is appealing on general grounds, it, however, currently remains largely unconstrained, notably because a fundamental understanding of the nonlinear asymptotic behavior of large-scale fluid magnetism in the astrophysically relevant but treacherous regime of large magnetic Reynolds number Rm is still lacking. We explore this problem using local high-resolution simulations of turbulent magnetohydrodynamics driven by an inhomogeneous helical forcing generating a sinusoidal profile of kinetic helicity, mimicking the hemispheric distribution of kinetic helicity in rotating turbulent fluid bodies. We identify a transition at large Rm to a nonlinear state, followed up to $\text{Rm}\simeq 3\times {10}^3$, consisting of strong, saturated small-scale magnetohydrodynamic turbulence and a weaker, traveling coherent large-scale field oscillation. This state is characterized by an asymptotically small resistive dissipation of magnetic helicity, by its spatial redistribution across the equator through turbulent fluxes driven by the hemispheric distribution of kinetic helicity, and by the tentative presence in the tangled dynamical magnetic field of plasmoids typical of reconnection at large Rm.

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• Astrophysical fluid dynamics
• Dynamo theory
• Magnetic field generation & plasma dynamo
• Magnetohydrodynamics