  Solar eruptive events such as coronal mass ejections and eruptive flares are
frequently associated with the emergence of magnetic flux from the convection
zone into the corona. We use three dimensional magnetohydrodynamic numerical
simulations to study the interaction of coronal magnetic fields with emerging
flux and determine the conditions that lead to eruptive activity. A simple
parameter study is performed, varying the relative angle between emerging
magnetic flux and a pre-existing coronal dipole field. We find that in all
cases, the emergence results in a sheared magnetic arcade that transitions to a
twisted coronal flux rope via low-lying magnetic reconnection. This structure,
however, is constrained by its own outer field, and so is non-eruptive in the
absence of reconnection with the overlying coronal field. The amount of this
overlying reconnection is determined by the relative angle between the emerged
and pre-existing fields. The reconnection between emerging and pre-existing
fields is necessary to generate sufficient expansion of the emerging structure
so that flare-like reconnection below the coronal flux rope becomes strong
enough to trigger its release. Our results imply that the relative angle is the
key parameter in determining whether the resultant active regions exhibit
eruptive behavior, and is thus a potentially useful candidate for predicting
eruptions in newly emerging active regions. More generally, our results
demonstrate that the detailed interaction between the convection
zone/photosphere and the corona must be calculated self-consistently in order
to model solar eruptions accurately.
