Driving outflows from young stars through the effects of internal disk fields

We examine the evolution of magnetized, differentially rotating buoyant ele ments ejected through the surface of an accretion disk into an unmagnetized ambient medium, using axisymmetric magnetohydrodynamic calculations. The e volution occurs in three distinct stages. First, angular momentum transfer along radial magnetic field lines allows part of each element to plunge tow ard the rotation axis. Next, the vertical gradient in total pressure accele rates some of the material at the axis upward to escape speed, forming a je t collimated by an azimuthal field. Finally, material near the base of the jet is brought close to solid-body rotation by Lorentz forces, the jet ceas es, and material subsequently ejected through the disk surface angles away from the axis and enters a magnetocentrifugal flow. Jets are produced over a large range in injection and Alfven speeds, while the magnetocentrifugal flows reach escape speed only when mass flux per field line is low. This me chanism may be useful in explaining the speeds, variability, and mass flow rates of jets and winds from protostars and T Tauri stars.