  We consider a three-dimensional bipolar force-free magnetic field with non
zero magnetic helicity, occupying a half-space, and study the problem of its
evolution driven by an imposed photospheric flux decrease. For this specific
setting of the Flux Cancellation Model describing coronal mass ejections
occuring in active regions, we address the issues of the physical meaning of
flux decrease, of the influence on field evolution of the size of the domain
over which this decrease is imposed, and of the existence of an energetic
criterion characterizing the possible onset of disruption of the configuration.
We show that: (1) The imposed flux disappearance can be interpreted in terms of
transport of positive and negative fluxes towards the inversion line, where
they get annihilated. (2) For the particular case actually computed, in which
the initial state is quite sheared, the formation of a twisted flux rope and
the subsequent global disruption of the configuration are obtained when the
flux has decreased by only a modest amount over a limited part of the whole
active region. (3) The disruption is produced when the magnetic energy becomes
of the order of the decreasing energy of a semi-open field, and then before
reaching the energy of the associated fully open field. This suggests that the
mechanism leading to the disruption is nonequilibrium as in the case where flux
is imposed to decrease over the whole region.
