arXiv:2602.13500v1 Announce Type: new
Abstract: Astrophysical and geophysical fluids commonly generate organized magnetic fields, despite having enormous magnetic Reynolds numbers $rm{Rm}$ and abundant small-scale turbulence. Flow-induced dynamo action produces these fields, with the “kinematic dynamo problem” devoted to determining the rate at which a flow exponentially amplifies weak magnetic fields. However, previous studies on high-Rm kinematic dynamos have generated flows via imposed volumetric forcing or oscillatory boundary conditions. In this letter, we investigate a system with three important attributes: realistic flow conditions, fast dynamo action (operational for $rm{Rm}toinfty$), and a subharmonic spatio-temporal structure. We show that unsteady Taylor–vortex flow, a regime observed in laboratory experiments, gives rise to fast dynamos with time and length scales twice those of the flow at high $rm{Rm}$. By numerically integrating a Floquet system driven by periodic oscillations of Taylor vortices, we solve the kinematic dynamo problem up to $rm{Rm} = 3.2 cdot 10^6$, calculating the dynamo’s growth rate as a function of Rm and streamwise wavenumber. We find the onset of instability and compute Finite-Time Lyapunov Exponents, which identify the regions of Lagrangian chaos required for fast dynamo action. To our knowledge, unsteady Taylor–vortex flow produces the most physically motivated fast dynamo to date.