  The evolution of a star is influenced by its internal rotation dynamics
through transport and mixing mechanisms, which are poorly understood. Magnetic
fields can play a role in transporting angular momentum and chemical elements,
but the origin of magnetism in radiative stellar layers is unclear. Using
global numerical simulations, we identify a subcritical transition to
turbulence due to the generation of a magnetic dynamo. Our results have many of
the properties of the theoretically-proposed Tayler-Spruit dynamo mechanism,
which strongly enhances transport of angular momentum in radiative zones. It
generates deep toroidal fields that are screened by the stellar outer layers.
This mechanism could produce strong magnetic fields inside radiative stars,
without an observable field on their surface.
