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Protoplanetary disks are believed to accrete onto their central T Tauri starbecause of magnetic stresses. Recently published shearing box simulationsindicate that Ohmic resistivity, ambipolar diffusion and the Hall effect allplay important roles in disk evolution. In the presence of a vertical magneticfield, the disk remains laminar between 1-5au, and a magnetocentrifugal diskwind forms that provides an important mechanism for removing angular momentum.Questions remain, however, about the establishment of a true physical windsolution in the shearing box simulations because of the symmetries inherent inthe local approximation. We present global MHD simulations of protoplanetarydisks that include Ohmic resistivity and ambipolar diffusion, where thetime-dependent gas-phase electron and ion fractions are computed under FUV andX-ray ionization with a simplified recombination chemistry. Our results showthat the disk remains laminar, and that a physical wind solution arisesnaturally in global disk models. The wind is sufficiently efficient to explainthe observed accretion rates. Furthermore, the ionization fraction atintermediate disk heights is large enough for magneto-rotational channel modesto grow and subsequently develop into belts of horizontal field. Depending onthe ionization fraction, these can remain quasi-global, or break-up intodiscrete islands of coherent field polarity. The disk models we present hereshow a dramatic departure from our earlier models including Ohmic resistivityonly. It will be important to examine how the Hall effect modifies theevolution, and to explore the influence this has on the observationalappearance of such systems, and on planet formation and migration.

Global simulations of protoplanetary disks with ohmic resistivity and ambipolar diffusion