Title: Origin of Weak Turbulence in the Outer Regions of Protoplanetary Disks,
Abstract: The mechanism behind angular momentum transport in protoplanetary disks, and
whether this transport is turbulent in nature, is a fundamental issue in planet
formation studies. Recent ALMA observations have suggested that turbulent
velocities in the outer regions of these disks are less than ~5-10% of the
sound speed, contradicting theoretical predictions of turbulence driven by the
magnetorotational instability (MRI). These observations have generally been
interpreted to be consistent with a large-scale laminar magnetic wind driving
accretion. Here, we carry out local, shearing box simulations with varying
ionization levels and background magnetic field strengths in order to determine
which parameters produce results consistent with observations. We find that
even when the background magnetic field launches a strong largely laminar wind,
significant turbulence persists and is driven by localized regions of vertical
magnetic field (the result of zonal flows) that are unstable to the MRI. The
only conditions for which we find turbulent velocities below the observational
limits are weak background magnetic fields and ionization levels well below
that usually assumed in theoretical studies. We interpret these findings within
the context of a preliminary model in which a large scale magnetic field,
confined to the inner disk, hinders ionizing sources from reaching large radial
distances, e.g., through a sufficiently dense wind. Thus, in addition to such a
wind, this model predicts that for disks with weakly turbulent outer regions,
the outer disk will have significantly reduced ionization levels compared to
standard models and will harbor only a weak vertical magnetic field.