  Non-ideal magnetohydrodynamical effects play a crucial role in determining
the mechanism and efficiency of angular momentum transport as well as the level
of turbulence in protoplanetary disks (PPDs), which are key to understanding
PPD evolution and planet formation. It was shown in our previous work that at 1
AU, the magnetorotational instability (MRI) is completely suppressed when both
Ohmic resistivity and ambipolar diffusion (AD) are taken into account,
resulting in a laminar flow with accretion driven by magnetocentrifugal wind.
In this work, we study the radial dependence of the laminar wind solution using
local shearing-box simulations. Scaling relation on the angular momentum
transport for the laminar wind is obtained, and we find that the wind-driven
accretion rate can be approximated as
M_dot~0.91x10^(-8)R_AU^(1.21)(B_z/10mG)^(0.93)M_Sun/yr, where B_z is the
strength of the large-scale vertical magnetic field threading the disk. The
result is independent of disk surface density. Four criteria are outlined for
the existence of the laminar wind solution: 1). Ohmic resistivity dominated
midplane region; 2). AD dominated disk upper layer; 3). Presence of (not too
weak) net vertical magnetic flux. 4). Sufficiently well ionized gas beyond disk
surface. All these criteria are likely to be met in the inner region of the
disk from ~0.3 AU to about 5-10 AU for typical PPD accretion rates. Beyond this
radius, angular momentum transport is likely to proceed due to a combination of
the MRI and disk wind, and eventually dominated by the MRI (in the presence of
strong AD) in the outer disk. Our simulation results provide key ingredients
for a new paradigm on the accretion processes in PPDs.
