  We present results from 3D magnetohydrodynamic (MHD) simulations of the
emergence of a twisted convection zone flux tube into a pre-existing coronal
dipole field. As in previous simulations, following the partial emergence of
the sub-surface flux into the corona, a combination of vortical motions and
internal magnetic reconnection forms a coronal flux rope. Then, in the
simulations presented here, external reconnection between the emerging field
and the pre-existing dipole coronal field allows further expansion of the
coronal flux rope into the corona. After sufficient expansion, internal
reconnection occurs beneath the coronal flux rope axis, and the flux rope
erupts up to the top boundary of the simulation domain ($\sim$ 36 Mm above the
surface). We find that the presence of a pre-existing field, orientated in a
direction to facilitate reconnection with the emerging field, is vital to the
fast rise of the coronal flux rope. The simulations shown in this paper are
able to self-consistently create many of the surface and coronal signatures
used by coronal mass ejection (CME) models. These signatures include: surface
shearing and rotational motions; quadrupolar geometry above the surface;
central sheared arcades reconnecting with oppositely orientated overlying
dipole fields; the formation of coronal flux ropes underlying potential coronal
field; and internal reconnection which resembles the classical flare
reconnection scenario. This suggests that proposed mechanisms for the
initiation of a CME, such as "magnetic breakout", are operating during the
emergence of new active regions.
