  We study the evolution of a magnetic arcade that is anchored to an accretion
disk and is sheared by the differential rotation of a Keplerian disk. By
including an extremely low external plasma pressure at large distances, we
obtain a sequence of axisymmetric magnetostatic equilibria and show that there
is a fundamental difference between field lines that are affected by the plasma
pressure and those are not (i.e., force-free). Force-free fields, while being
twisted by the differential rotation of the disk, expand outward at an angle of
$\sim 60^\circ$ away from the rotation axis, consistent with the previous
studies. These force-free field lines, however, are enclosed by the outer field
lines which originate from small disk radii and come back to the disk at large
radii. These outer fields experience most of the twist, and they are also
affected most by the external plasma pressure. At large cylindrical radial
distances, magnetic pressure and plasma pressure are comparable so that any
further radial expansion of magnetic fields is prevented or slowed down greatly
by this pressure. This hindrance to cylindrical radial expansion causes most of
the added twist to be distributed on the ascending portion of the field lines,
close to the rotation axis. Since these field lines are twisted most, the
increasing ratio of the toroidal $B_{\phi}$ component to the poloidal component
$B_{R,z}$ eventually results in the collimation of magnetic energy and flux
around the rotation axis. We discuss the difficulty with adding a large number
of twists within the limitations of the magnetostatic approximation.
