The life-cycle of the jet-driven shear-flow dynamo

Shear flows can generate and sustain large-scale, quasi-cyclic, self-organized magnetic fields in three dimensions via a process called the dynamo. Here, the essential steps of a dynamo process are identified and confirmed using energy transfer analyses of turbulence driven by the Kelvin–Helmholtz (KH) instability. The dynamo cycle begins with an (x, y)-averaged mean horizontal shear-flow U_x(z), which is maintained externally. The KH instability, which represents the x-varying fluctuations, nonlinearly excites x-invariant, y-varying vertical flows and magnetic fields—labeled u-rolls and b-rolls. These vertical perturbations are strained by the mean shear flow to generate horizontal x-invariant, y-varying flows and magnetic fields. These are labeled zonal jets and zonal magnetic fields. The zonal jets then stretch the b-rolls, creating an (x, y)-averaged mean horizontal magnetic field. The z-gradient of the mean magnetic field saturates when the u-rolls advect the mean magnetic field vertically. The zonal jets are sustained by energy transfer from the primary KH instability of the mean shear flow. These processes are robust under key parameter variations. This dynamo cycle explains the operation mechanism of a recently confirmed mean-field dynamo theory—the mean-vorticity effect—applicable in a host of plasmas.