Turbulent Dynamos Driven by Flow-Instability-Induced Topology-Favored Zonal Jets*

Large-scale magnetic fields in the universe are thought to require helical flows, though they are quenched by Lorentz feedback. Since previous non-helical dynamo mechanisms are limited, we present here a robust new mechanism of large-scale magnetic-field generation, via a weakly magnetized shear-flow instability. In 3D, with a mean inhomogeneous shear flow U_x(z), x-varying perturbations are unstable and nonlinearly excite x-invariant, y-periodic stable perturbations. The unique topology of such perturbations protects them against mean-flow shearing. Instead, they are favorably stretched by the mean flow to form strong zonal jets and zonal magnetic fields, of the form ∂_zU_x. The jets stretch field fluctuations, generating a mean field with z-reversed polarity. Long-time locking and short-time slipping of jet-field cross phase lead to a sustained dynamo and mean-field polarity reversals. An analytical model captures these mechanisms and predicts the reversed mean-field profiles, mean-field reversal times, and their trends with changes in magnetic Prandtl and Alfvenic Mach numbers. A mean-field dynamo description shows the traditional helicity effect is unimportant, consistent with the mean fields being far from a force-free Taylor state. Relaxation analysis of the field configuration, a topic dear to R. Dewar, confirms that the zonal-flow dynamo arises from a balance between turbulent diffusion and the mean-vorticity effect. Signatures of such dynamos in nature and laboratory are discussed.