  In Orion BN/KL, proper motions of 7 mm vibrationally-excited SiO masers trace
rotation of a nearly edge-on disk and a bipolar wide-angle outflow 10-100 AU
from radio Source I, a binary young stellar object (YSO) of ~20 Msun. Here we
map ground-state 7 mm SiO emission with the Very Large Array and track proper
motions over 9 years. The innermost and strongest emission lies in two extended
arcs bracketing Source I. The proper motions trace a northeast-southwest
bipolar outflow 100-1000 AU from Source I with a median 3D motion of ~18 km/s.
An overlying distribution of 1.3 cm H2O masers betrays similar flow
characteristics. Gas dynamics and emission morphology traced by the masers
suggest the presence of a magnetocentrifugal disk-wind. Reinforcing evidence
lies in the colinearity of the flow, apparent rotation across the flow parallel
to the disk rotation, and recollimation that narrows the flow opening angle
~120 AU downstream. The arcs of ground-state SiO emission may mark the
transition point to a shocked super-Alfvenic outflow.
