  The structure of electric current and magnetic helicity in the solar corona
is closely linked to solar activity over the 11-year cycle, yet is poorly
understood. As an alternative to traditional current-free "potential field"
extrapolations, we investigate a model for the global coronal magnetic field
which is non-potential and time-dependent, following the build-up and transport
of magnetic helicity due to flux emergence and large-scale photospheric
motions. This helicity concentrates into twisted magnetic flux ropes, which may
lose equilibrium and be ejected. Here, we consider how the magnetic structure
predicted by this model-in particular the flux ropes-varies over the solar
activity cycle, based on photospheric input data from six periods of cycle 23.
The number of flux ropes doubles from minimum to maximum, following the total
length of photospheric polarity inversion lines. However, the number of flux
rope ejections increases by a factor of eight, following the emergence rate of
active regions. This is broadly consistent with the observed cycle modulation
of coronal mass ejections, although the actual rate of ejections in the
simulation is about a fifth of the rate of observed events. The model predicts
that, even at minimum, differential rotation will produce sheared,
non-potential, magnetic structure at all latitudes.
