On the factors determining the eruptive character of solar flares

Abstract

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 $d_{\mathrm{FC}}$) 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, $d_{\mathrm{PC}}$. Flares originating from underneath the AR dipole $(d_{\mathrm{FC}}/d_{\mathrm{PC}}<0.5$) tend to be eruptive if launched from compact ARs ($d_{\mathrm{PC}}\le 60$ Mm) and confined if launched from extended ARs. Flares ejected from the periphery of ARs ($d_{\mathrm{FC}}/d_{\mathrm{PC}}>0.5$) are predominantly eruptive. In confined events the flare-relevant field adjusts its orientation quickly to that of the underlying dipole with height ($\Delta \phi \gtrsim 40^{\circ }$ 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, $h_{\mathrm{crit}}=h(n=1.5)$, discriminates best between confined ($h_{\mathrm{crit}}\gtrsim 40$ Mm) and eruptive flares ($h_{\mathrm{crit}}\lesssim 40$ Mm). It discriminates better than $\Delta \phi$, implying that the decay of the confining field plays a stronger role than its orientation at different heights.

17 pages, 7 figures, accepted for publication in ApJ

References in corpus (5)

• Torus instability
• The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: Overview and Performance
• Magnetic Properties of Solar Active Regions that Govern Large Solar Flares and Eruptions
• Numerical simulations of fast and slow coronal mass ejections
• Critical Height of the Torus Instability in Two-Ribbon Solar Flares

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