Coronal mass ejection initiation: On the nature of the Flux Cancellation Model

TL;DR

This paper investigates the Flux Cancellation Model for coronal mass ejections, analyzing how flux decrease influences magnetic field evolution, formation of flux ropes, and the energetic conditions leading to eruption in active regions.

Contribution

It provides a detailed physical interpretation of flux decrease, examines the impact of domain size on evolution, and identifies an energetic criterion for eruption onset in a 3D force-free magnetic field.

Findings

Flux disappearance corresponds to flux transport and annihilation at the inversion line.

Formation of a twisted flux rope occurs with modest flux decrease in a sheared initial state.

Disruption occurs when magnetic energy drops to the order of a semi-open field energy.

Abstract

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.