CORONAL MASS EJECTION INITIATION AND COMPLEX TOPOLOGY CONFIGURATIONS IN THE FLUX CANCELLATION AND BREAKOUT MODELS

摘要

We present some new results showing that the flux cancellation model for coronal mass ejections (CMEs) works well also in a complex-topology magnetic field. We consider as a model problem the case of the flux-cancellation-driven evolution of a quadrupolar configuration. We find that (1) during the first phase, the field evolves slowly, with a twisted flux rope in equilibrium being created at some time; (2) nonequilibrium sets in at a critical time and the configuration experiences a major global disruption. These features are similar to those previously obtained for a bipolar configuration. Some differences between the two cases are however observed: (1) the presence of an X-point above the twisted flux rope makes the expulsion of the latter much easier due to the weaker confinement near this point; this difference may be at the origin of the existence of two classes of CMEs—fast and slow; (2) the energy W(t) of the configuration remains smaller than the energy W_σ(t) of the associated totally open field, and then the disruption does not occur when W(t) ～ Wσ(t), as in the bipolar case. Rather we get nonequilibrium when W(t) ～ W_(so)(t), where W_(SO)(t) is the energy of a semiopen field which has its open lines connected to the two central spots on which flux cancellation is imposed. A consequence of our results is that the topological complexity of a preempting configuration cannot be taken as a criterion for eliminating the flux cancellation model in favor of the well-known breakout model.