Simulations of a Magnetic Fluctuation Driven Large Scale Dynamo and Comparison with a Two-scale Model

Abstract

Models of large scale (magnetohydrodynamic) dynamos (LSD) which couple large scale field growth to total magnetic helicity evolution best predict the saturation of LSDs seen in simulations. For the simplest so called "α2" LSDs in periodic boxes, the electromotive force driving LSD growth depends on the difference between the time-integrated kinetic and current helicity associated with fluctuations. When the system is helically kinetically forced (KF), the growth of the large scale helical field is accompanied by growth of small scale magnetic (and current) helicity which ultimately quench the LSD. Here, using both simulations and theory, we study the complementary magnetically forced(MF) case in which the system is forced with an electric field that supplies magnetic helicity. For this MF case, the kinetic helicity becomes the back-reactor that saturates the LSD. Simulations of both MF and KF cases can be approximately modeled with the same equations of magnetic helicity evolution, but with complementary initial conditions. A key difference between KF and MF cases is that the helical large scale field in the MF case grows with the same sign of injected magnetic helicity, whereas the large and small scale magnetic helicities grow with opposite sign for the KF case. The MF case can arise even when the thermal pressure is approximately smaller than the magnetic pressure, and requires only that helical small scale magnetic fluctuations dominate helical velocity fluctuations in LSD driving. We suggest that LSDs in accretion discs and Babcock models of the solar dynamo are actually MF LSDs.

12 pages, 34 figures

References in corpus (3)

• Minimal tau approximation and simulations of the alpha effect
• Magnetorotational instability driven dynamos at low magnetic Prandtl numbers
• Toward Coupling Flow Driven and Magnetically Driven Dynamos

Cited by in corpus (15)

• Advances in mean-field dynamo theory and applications to astrophysical turbulence
• Magnetic Helicity and Large Scale Magnetic Fields: A Primer
• The magnetic shear-current effect: generation of large-scale magnetic fields by the small-scale dynamo
• Accretion Disks and Dynamos: Toward a Unified Mean Field Theory
• Coherent nonhelical shear dynamos driven by magnetic fluctuations at low Reynolds numbers
• E and B polarizations from inhomogeneous and solar surface turbulence
• On the resilience of helical magnetic fields to turbulent diffusion and the astrophysical implications
• On the Inverse Transfer of (Non-)Helical Magnetic Energy in a Decaying Magnetohydrodynamic Turbulence
• Negative Magnetic Diffusivity beta replacing alpha effect in Helical Dynamo
• Ribbons characterize magnetohydrodynamic magnetic fields better than lines: a lesson from dynamo theory
• Influence of small scale $E_M$ and $H_M$ on the growth of large scale magnetic field
• Magnetic Effect on Potential Barrier for Nucleosynthesis II
• Influence of initial conditions on the large-scale dynamo growth rate
• Principle of Helical \& Nonhelical Dynamo and $\alpha$ effect in Field Structure model
• $\alpha$ Effect and Magnetic Diffusivity $\beta$ in Helical Plasma under Turbulence Growth