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“Magnetic Fields in Protoplanetary Disks” Dr. Mark Wardle Macquarie University Sydney, Australia |
| Magnetic fields may play a key role in the dynamics and evolution of protoplanetary disks by efficiently transporting angular momentum by MHD turbulence or via the magnetocentrifugal acceleration of outflows from the disk surface. Magnetically-driven mixing has implications for disk chemistry and the evolution of the grain population. In addition magnetic turbulence modifies the effective viscous response of the disk, which determines whether planets migrate inwards or outwards. However, the weak ionisation of protoplanetary disks means that magnetic fields may not be able to effectively couple to the matter. Dust grains tend to supress magnetic coupling by soaking up electrons and ions. However, once grains have grown to a few microns in size their effect starts to wane and magnetic fields can couple to the gas even at the disk midplane. The Hall effect tends to dominate the diffusion of the magnetic field except at the disk surfaces where the low density of neutrals permits the ions to remain attached to the field lines. Implications for the dynamics and evolution of protoplanetary discs are discussed. |