Dynamos in Shear Flows: The Dual Role of Alfvénization

Magnetic fields in nature are generated and sustained by turbulent flows—an effect called the dynamo. Traditional dynamos rely on helical fluid motions acting on an imposed mean field; however, this mean-field effect is reduced to near zero because of Alfvénization—a key process which creates velocity and magnetic fields that are equipartitioned and aligned. Here, we show that Alfvénization—which inhibits traditional dynamos—enables a less-familiar dynamo mechanism via large-scale vortical motions. Considering an unstable and driven shear flow, we simulate three-dimensional MHD turbulence with up to 4096 × 4096 × 8192 grid points. Turbulence generates quasi-periodic, large-scale magnetic fields from zero initial large-scale field. This generation is due to the mean-vorticity effect—which was postulated in [1] and validated here. To unveil this effect, we develop analytic theory, its physical interpretation, and its working mechanism. The mechanism is general and robust, as the only required ingredient is the presence of an inhomogeneous shear flow; the flow spontaneously generates turbulence and an exponentially growing dynamo. Central in this mechanism are inherently three-dimensional jets, which are exact nonlinear solutions to the magnetohydrodynamic equations. The key features of the jet-driven dynamo agree with laboratory dynamo experiments [2] and carry implications for stellar dynamos and binary neutron star mergers, among others.



[1] PoF B 2, 1589 (1990).

[2] ApJ 759, 80 (2012).