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The mechanism behind angular momentum transport in protoplanetary disks, andwhether this transport is turbulent in nature, is a fundamental issue in planetformation studies. Recent ALMA observations have suggested that turbulentvelocities in the outer regions of these disks are less than ~5-10% of thesound speed, contradicting theoretical predictions of turbulence driven by themagnetorotational instability (MRI). These observations have generally beeninterpreted to be consistent with a large-scale laminar magnetic wind drivingaccretion. Here, we carry out local, shearing box simulations with varyingionization levels and background magnetic field strengths in order to determinewhich parameters produce results consistent with observations. We find thateven when the background magnetic field launches a strong largely laminar wind,significant turbulence persists and is driven by localized regions of verticalmagnetic field (the result of zonal flows) that are unstable to the MRI. Theonly conditions for which we find turbulent velocities below the observationallimits are weak background magnetic fields and ionization levels well belowthat usually assumed in theoretical studies. We interpret these findings withinthe context of a preliminary model in which a large scale magnetic field,confined to the inner disk, hinders ionizing sources from reaching large radialdistances, e.g., through a sufficiently dense wind. Thus, in addition to such awind, this model predicts that for disks with weakly turbulent outer regions,the outer disk will have significantly reduced ionization levels compared tostandard models and will harbor only a weak vertical magnetic field.

Origin of Weak Turbulence in the Outer Regions of Protoplanetary Disks