  We present the results of a set of three-dimensional numerical simulations of
magnetic flux emergence from below the photosphere into the corona that include
a uniform and horizontal coronal magnetic field mimicking a pre-existing
large-scale coronal magnetic system. Cases with different relative orientations
of the upcoming and coronal fields are studied. Upon contact, a concentrated
current sheet with the shape of an arch or bridge is formed at the interface
which marks the positions of maximum jump in the field vector between the two
systems. Relative angles above 90 degrees yield abundant magnetic reconnection
and plasma heating. The reconnection is seen to be intrisincally
three-dimensional in nature, except at singular positions along the current
sheet. It drives collimated high-speed and high-temperature outflows only a
short distance from the reconnection site that propagate along the ambient
magnetic field lines as jets. Due to the low plasma density in the corona,
these jets may propagate over large distances and, therefore help distribute
high-density and high-temperature plasma along these newly reconnected field
lines. The experiments permit to discern and visualize the three-dimensional
shape and relative position of the upcoming plasma hill, high-speed jets and
subphotospheric flux system. As a result of the reconnection, magnetic field
lines from the magnetized plasma below the surface end up as coronal field
lines, thus causing a profound change in the connectivity of the magnetic
regions in the corona. The experiments presented here thus yield a number of
features repeatedly observed with the TRACE satellite and the YOHKOH-SXT
detector, like the establishment of connectivity between emergent and
pre-existing active regions, local heating and high-velocity outflows.
