Invited review abstract

The magnetic environment and evolution in flares: General theory and flux emergence models
Bernhard Kliem


Developing a proper description of the magnetic environment and evolution in eruptive solar events -- flares, coronal mass ejections and prominence/filament eruptions -- requires us to understand the force-free equilibrium configurations that store the free energy available to power the eruption. This talk will focus at their topology and stability, not so much at their formation. I will emphasize the view at the geometry of current flow in the corona and discuss at some length the Titov & Demoulin model, a single current loop arching up into the corona, and van Ballegooijen's model, a hollow current channel running flat in a developed filament channel. For both, we have some quantitative knowledge where they become unstable -- the only quantitative CME onset conditions so far established for 3D configurations, but only for the T&D configuration we also have established models of the relevant MHD instabilities.

One interesting aspect of these equilibria is the topology of the quasi-separatrix layers (QSLs). I will present evidence of the occurrence of a bald-patch separatrix surface (O-type topology) obtained from sigmoid observations and evidence of the occurrence of a hyperbolic flux tube (X-type topology) obtained from the CME-flare relationship.

The considerations of stability of these equilibria serve as a guide in understanding the role of newly emerging flux in triggering an eruption. Most of the work on this subject seems to lie ahead. While it is relatively clear that emergence away from the neutral line can weaken the overlying flux if it reconnects with that flux, we still have to understand how flux emerging at the neutral line destabilizes the configuration irrespective of its orientation. The problem possesses an incredible intrinsic complexity, due to the multi-dimensionality of the relevant parameter space and due to the richness of possible dynamic behavior enabled by magnetic reconnection.

Turning to the evolution of the magnetic field in the course of eruptions, I will show a simulation that produces a "footpoint exchange" of the erupting flux and present simulations of the bursty mode of reconnection in the flare current sheet. The latter may be relevant to the scales of particle acceleration and to the irregular supra-arcade downflows observed in some events. The possibility of reconnection above the erupting flux will also be illustrated.