Angular momentum transport in weakly ionized protoplanetary disks
          Raquel Salmeron, University of Chicago
          Mark Wardle, Macquarie University
          Arieh Koenigl, University of Chicago
          Magnetic fields play important roles in the dynamics and evolution of protoplanetary disks. They are thought to generate MHD turbulence and drive the acceleration and collimation of outflows frequently observed in such systems. This magnetic activity may effectively regulate the evolution of the `accretion phase' of disks by providing mechanisms that remove angular momentum from the gas, enabling most of it to be accreted. In the weakly ionized environment of such disks, however, the ionization fraction is not enough to produce good magnetic coupling over their entire vertical and radial extention. As a result, the ability of the magnetic field to effectively couple to the fluid -and drive these magnetic processes- is strongly dependent on the conductivity of the gas and its spatial dependency.
    We present two examples of magnetic activity in weakly ionized disks: The vertical structure and linear growth of the magnetorotational instability (MRI) and the launching of magnetocentrifugally driven winds. Our method incorporates a realistic ionization profile, with and without dust grains, and the effect of Hall diffusion. Results indicate that, despite the low magnetic coupling, magnetic fields are dynamically important over a broad range of fluid conditions and field strengths. At 1 AU, without grains, MRI-unstable modes exist for field strengths of up to a few gauss. When grains are assumed to be mixed with the gas, the MRI is active for field strengths of up to a fraction of a gauss. The implications of these results to the evolution of protoplanetary disks are briefly discussed.