  Coronal mass ejections (CMEs) are the primary drivers of severe space weather
disturbances in the heliosphere. Models of CME dynamics have been proposed that
do not fully include the effects of magnetic reconnection on the forces driving
the ejection. Both observations and numerical modeling, however, suggest that
reconnection likely plays a major role in most, if not all, fast CMEs. Here, we
theoretically investigate the accretion of magnetic flux onto a rising ejection
by reconnection involving the ejection's background field. This reconnection
alters the magnetic structure of the ejection and its environment, thereby
modifying the forces acting upon the ejection, generically increasing its
upward acceleration. The modified forces, in turn, can more strongly drive the
reconnection. This feedback process acts, effectively, as an instability, which
we refer to as a reconnective instability. Our analysis implies that CME models
that neglect the effects of reconnection cannot accurately describe observed
CME dynamics. Our ultimate aim is to understand changes in CME acceleration in
terms of observable properties of magnetic reconnection, such as the amount of
reconnected flux. This flux can be estimated from observations of flare ribbons
and photospheric magnetic fields.
