Large-scale dynamos driven by shear-flow-induced jets.

Abstract

At every scale they occupy, magnetic fields affect various phenomena, including star formation, cosmic-ray transport, charged-particle acceleration, space weather, transport in planetary atmospheres and laboratory plasmas. These fields are often generated and sustained by turbulent flows in a process called the dynamo. In 1955, E. N. Parker parameterized the effects of small-scale turbulence to propose a mean-field dynamo theory¹. The widely used theory reproduces observed large-scale fields but suffers from difficulty in tuning parameters as they are not justified from first principles: studies of turbulent flows show tangled magnetic fields, which are folded and fragmented into small-scale structures owing to shear-flow straining^2,3. Here, considering a shear flow that is unstable and driven, we develop analytic theory and perform three-dimensional, advanced computer simulations of turbulence with up to 4,096 × 4,096 × 8,192 grid points, showing ab initio generation of quasi-periodic, large-scale magnetic fields. The generation occurs via the mean-vorticity effect-an additional mean-field dynamo process postulated⁴ in 1990. Crucial to this dynamo is the prior generation of large-scale three-dimensional jets, robustly produced as topologically protected and exact nonlinear solutions of the magnetohydrodynamic equations. The jet-driven dynamo applies to shear-driven laboratory and astrophysical systems. These include binary neutron star mergers^5,6, where the reported dynamo probably operates on microsecond timescales to produce in milliseconds some of the strongest magnetic fields in the Universe⁷, providing signals for multi-messenger astronomy⁸.