Formation of 3D nullpoint topologies, torus-unstable flux ropes, and erupting sigmoids in the solar corona

Tibor Torok (Observatory of Paris Meudon)

In this talk I will present two recent studies which have been undertaken in collaboration with the SSXG group at SAO. The first study addressed the formation of 3D nullpoint topologies in the solar corona by combining Hinode/XRT observations of a small dynamic limb event, which occurred beside a non-erupting prominence cavity, with a 3D zero beta MHD simulation. To this end, we model the boundary-driven 'kinematic' emergence of a compact, intense, and uniformly twisted flux tube into a potential field arcade that overlies a weakly twisted coronal flux rope. The expansion of the emerging flux in the corona gives rise to the formation of a nullpoint at the interface of the emerging and the pre-existing fields. We unveil a two-step reconnection process at the nullpoint that eventually yields the formation of a broad 3D fan-spine configuration above the emerging bipole. The first reconnection involves emerging fields and a set of large-scale arcade field lines. It results in the launch of a torsional MHD wave that propagates along the arcades, and in the formation of a sheared loop system on one side of the emerging flux. The second reconnection occurs between these newly formed loops and remote arcade fields, and yields the formation of a second loop system on the opposite side of the emerging flux. The two loop systems collectively display an anenome pattern that is located below the fan surface. The nature and timing of the features which occur in the simulation do qualititatively reproduce those observed by XRT in the particular event studied. Moreover, the two-step reconnection process suggests a new consistent and generic model for the formation of anemone regions in the solar corona.

In the second study, we analyzed the physical mechanisms that form a 3D coronal flux rope and cause its eruption, using a zero beta MHD simulation of an initially potential bipolar field that evolves by means of simultaneous slow magnetic field diffusion and shearing motions in the photosphere. As in similar models, flux cancellation driven photospheric reconnection in a bald-patch (BP) separatrix transforms the sheared arcades into a slowly rising stable flux rope. A transition from a BP to a quasi-separatrix layer (QSL) topology occurs later on in the evolution, while the flux rope keeps growing and slowly rising, now due to coronal tether-cutting reconnection. As the rope reaches the altitude at which the overlying field drops sufficiently fast for the onset of the ideal MHD torus instability, it starts to accelerate rapidly upward. Thus we find that photospheric flux-cancellation and tether-cutting coronal reconnection do not trigger CMEs in bipolar magnetic fields, but are key pre-eruptive mechanisms for flux ropes to build up and to rise to the critical height above the photosphere at which the torus instability causes the eruption. Simplified synthetic soft X-ray images, obtained from the distribution of the electric currents in the simulation, allowed us a qualitative comparison with an erupting sigmoid recently observed by Hinode/XRT, which will be briefly discussed.