On the Factors Determining the Eruptive Character of Solar Flares

We investigated how the magnetic field in solar active regions (ARs) controls flare activity, i.e., whether a confined or eruptive flare occurs. We analyzed 44 flares of GOES class M5.0 and larger that occurred during 2011-2015. We used 3D potential magnetic field models to study their location (using the flare distance from the flux-weighted AR center d(FC)) and the strength of the magnetic field in the corona above (via decay index n and flux ratio). We also present a first systematic study of the orientation of the coronal magnetic field, using the orientation phi of the flare-relevant polarity inversion line as a measure. We analyzed all quantities with respect to the size of the underlying dipole field, characterized by the distance between the opposite-polarity centers, d(PC). Flares originating from underneath the AR dipole (d(FC)/d(PC). < 0.5) tend to be eruptive if launched from compact ARs (d(PC) <= 60Mm) and confined if launched from extended ARs. Flares ejected from the periphery of ARs (d(FC)/d(PC) > 0.5) are predominantly eruptive. In confined events, the flare-relevant field adjusts its orientation quickly to that of the underlying dipole with height (Delta phi greater than or similar to 40 degrees until the apex of the dipole field), in contrast to eruptive events where it changes more slowly with height. The critical height for torus instability, h(crit) = h(n = 1.5), discriminates best between confined (h(crit) greater than or similar to 40Mm) and eruptive flares (h(crit) less than or similar to 40Mm). It discriminates better than Delta phi, implying that the decay of the confining field plays a stronger role than its orientation at different heights.