  It is generally believed that the magnetic free energy accumulated in the
corona serves as a main energy source for solar explosions such as coronal mass
ejections (CMEs). In the framework of the flux rope catastrophe model for CMEs,
the energy may be abruptly released either by an ideal magnetohydrodynamic
(MHD) catastrophe, which belongs to a global magnetic topological instability
of the system, or by a fast magnetic reconnection across preexisting or
rapidly-developing electric current sheets. Both ways of magnetic energy
release are thought to be important to CME dynamics. To disentangle their
contributions, we construct a flux rope catastrophe model in the corona and
solar wind and compare different cases in which we either prohibit or allow
magnetic reconnection to take place across rapidly-growing current sheets
during the eruption. It is demonstrated that CMEs, even fast ones, can be
produced taking the ideal MHD catastrophe as the only process of magnetic
energy release. Nevertheless, the eruptive speed can be significantly enhanced
after magnetic reconnection sets in. In addition, a smooth transition from slow
to fast eruptions is observed when increasing the strength of the background
magnetic field, simply because in a stronger field there is more free magnetic
energy at the catastrophic point available to be released during an eruption.
This suggests that fast and slow CMEs may have an identical driving mechanism.
