A Self-Consistent Framework for Nonhelical Dynamos in MRI Turbulence: The Rotation-Shear-Current Effect

Astrophysical accretion disks often exhibit large-scale magnetic fields even in the absence of helicity or vertical stratification. Using direct numerical simulations, we identify the physical mechanism by which such nonhelical dynamos operate in unstratified, zero-net-flux, magnetorotationally unstable turbulence. In this process, fluctuating magnetic tension drives turbulence and generates velocity fluctuations. These are reoriented by the Coriolis force into a pattern that correlates with magnetic fluctuations, producing a mean electromotive force (EMF). The resulting EMF amplifies large-scale radial magnetic fields, which are then stretched by background shear into azimuthal fields, completing the dynamo loop.

We demonstrate that this self-sustaining mechanism is governed by a consistently negative off-diagonal component of the turbulent diffusivity tensor, confirming a “rotation-shear-current” dynamo. Unlike traditional models, our approach constructs the EMF directly from the evolution of velocity-magnetic correlations, without relying on closure approximations. Finally, we identify the saturation mechanism: nonlinear feedback from Lorentz force fluctuations generates third-order correlations that counteract the dynamo growth. These results provide a comprehensive physical picture of how large-scale magnetic fields are generated and regulated in magnetorotational turbulence.