Assembling the Ingredients for a Jet: How Do Large-Scale Magnetic Fields Get There and What Happens When They Do?

Abstract

Jets and outflows are observed around a wide variety of accreting objects and seem to be a near-ubiquitous feature of accretion disks. Large-scale magnetic fields are thought to be necessary for jet formation in many systems, but a longstanding puzzle is that the turbulence which is responsible for inward disk accretion should be even more efficient at causing a large-scale magnetic field to diffuse outward; it just doesn't seem possible to build up a strong field in the inner disk through advection of a weak one from outside. Here, we report theoretical work which challenges this conventional wisdom and shows that the surface layer of the disk (which is nonturbulent) can easily advect magnetic fields inward. This opens up the possibility for more diverse and robust jet formation than is seen in numerical simulations which assume that the large-scale field must be generated entirely via a dynamo within the inner disk. In addition, strong disk turbulence (as inferred from observations) can naturally be explained by the magnetorotational instability feeding off a large-scale magnetic field anchored in the surface layer.