Triggering of eruptive flares: Destabilization of the preflare magnetic field configuration

Abstract

This paper takes the three-dimensional configuration of the magnetic field in and before eruptive flares as our main guide to how the preflare field comes to lose its stability and erupt. From observed characteristics (1) of the preflare magnetic field configuration, (2) of the onset and development of the eruption of this configuration before and during the flare, and (3) of the onset and development of the flare energy release (i.e., the heating and particle acceleration) within the erupting field, the typical erupting field configuration for two-ribbon eruptive flares is constructed. The observational centerpiece for this construction is the evidence from the Marshall Space Flight Center vector magnetograph that strong magnetic shear along the main magnetic inversion line is critical for large eruptive flares. From (a) the empirical field configuration and (b) the observation that the initial flare brightening typically stems from points where opposite-polarity flux is gradually merging and canceling at or near the main inversion line, it is argued (1) that eruptive flares are driven by the eruptive expansion of the strongly sheared core of the preflare magnetic field, (2) that this eruption is triggered by preflare slow reconnection accompanying flux cancellation in the sheared core, and (3) that in some flares the triggering reconnection and flux cancellation is between opposite-polarity strands of the extant preflare sheared core field, while in other flares it is between the sheared core field and new emerging flux.