  We consider the conditions under which a rotating magnetic object can produce
a magnetically powered outflow in an initially unmagnetized medium stratified
under gravity. 3D MHD simulations are presented in which the footpoints of
localized, arcade-shaped magnetic fields are put into rotation. It is shown how
the effectiveness in producing a collimated magnetically powered outflow
depends on the rotation rate, the strength and the geometry of the field. The
flows produced by uniformly rotating, non-axisymmetric fields are found to
consist mainly of buoyant plumes heated by dissipation of rotational energy.
Collimated magnetically powered flows are formed if the field and the rotating
surface are arranged such that a toroidal magnetic field is produced. This
requires a differential rotation of the arcades' footpoints. Such jets are
well-collimated; we follow their propagation through the stratified atmosphere
over 100 times the source size. The magnetic field is tightly wound and its
propagation is dominated by the development of non-axisymmetric instabilities.
We observe a Poynting flux conversion efficiency of over 75% in the longest
simulations. Applications to the collapsar model and protostellar jets are
discussed.
