Magnetic fields in protoplanetary disks
Freeman, John W., Jr.
Doctor of Philosophy
We study the origin, configuration and effects of magnetic fields in protoplanetary disks. Standard accretion disk models are adopted for protoplanetary disks to determine their electrical properties. A new computational approach to calculate the two-dimensional large scale magnetic field in thin disks, is used to study two scenarios previously proposed for the origin of magnetic fields in protoplanetary disks. We first consider the possibility that the accretion flow in a protoplanetary disk drags an external, uniform and vertical magnetic field to the required configuration to launch winds centrifugally. Results depend strongly on the magnetic Prandtl number of the prescribed turbulent motions in the disk. For fiducial values of such parameter, magnetic field dragging is unlikely to yield a configuration capable of driving cold winds centrifugally. For this to happen, the Prandtl number must be reduced by almost two orders of magnitude from the expected value, and still dragging in the disk's outer portions will be ineffective. In the second part of this thesis we calculate the magnetic configuration from an $\alpha\Omega$ dynamo operating inside a protoplanetary disk. A vacuum is assumed outside the disk. We incorporate a saturation mechanism for the dynamo instability to model the back reaction of the Lorentz force on the turbulent motions. This allows us to study the feasibility of achieving a wind-conducive magnetic configuration from the interaction of the dynamo field and a weak, externally generated, magnetic field. In general, our results indicate that some combinations of disk models and exterior magnetic field strengths result in portions of the disk being threaded by open field lines with the right configuration to drive winds. In summary, dynamo magnetic fields may be sustained in extended portions of protoplanetary disks for times comparable to the lifetime of the disk. However, the existence of an intermediate region, where the low ionization does not allow the field to be regenerated, is a general feature of viscous protoplanetary disk models. The contribution of the generated magnetic fields to the transport of angular momentum through the accretion disk, can be comparable to the effect of the turbulent viscosity.