  Loss of equilibrium of magnetic flux ropes is a leading candidate for the
origin of solar coronal mass ejections (CMEs). The aim of this paper is to
explore to what extent this mechanism can account for the initiation of CMEs in
the global context. A simplified MHD model for the global coronal magnetic
field evolution in response to flux emergence and shearing by large-scale
surface motions is described and motivated. Using automated algorithms for
detecting flux ropes and ejections in the global magnetic model, the effects of
key simulation parameters on the formation of flux ropes and the number of
ejections are considered, over a 177-day period in 1999. These key parameters
include the magnitude and sign of magnetic helicity emerging in active regions,
and coronal diffusion. The number of flux ropes found in the simulation at any
one time fluctuates between about 28 and 48, sustained by the emergence of new
bipolar regions, but with no systematic dependence on the helicity of these
regions. However, the emerging helicity does affect the rate of flux rope
ejections, which doubles from 0.67 per day if the bipoles emerge untwisted to
1.28 per day in the run with greatest emerging twist. The number of ejections
in the simulation is also increased by 20%-30% by choosing the majority sign of
emerging bipole helicity in each hemisphere, or by halving the turbulent
diffusivity in the corona. For reasonable parameter choices, the model produces
approximately 50% of the observed CME rate. This indicates that the formation
and loss of equilibrium of flux ropes may be a key element in explaining a
significant fraction of observed CMEs.
