Critical Height of the Torus Instability in Two-Ribbon Solar Flares

Abstract

We studied the background field for 60 two-ribbon flares of M-and-above classes during 2011--2015. These flares are categorized into two groups, i.e., \emph{eruptive} and \emph{confined} flares, based on whether a flare is associated with a coronal mass ejection or not. The background field of source active regions is approximated by a potential field extrapolated from the $B_z$ component of vector magnetograms provided by the Helioseismic and Magnetic Imager. We calculated the decay index $n$ of the background field above the flaring polarity inversion line, and defined a critical height $h_{\mathrm{crit}}$ corresponding to the theoretical threshold ($n_{\mathrm{crit}}=1.5$) of the torus instability. We found that $h_{\mathrm{crit}}$ is approximately half of the distance between the centroids of opposite polarities in active regions, and that the distribution of $h_{\mathrm{crit}}$ is bimodal: it is significantly higher for confined flares than for eruptive ones. The decay index increases monotonously with increasing height for 86\% (84\%) of the eruptive (confined) flares but displays a saddle-like profile for the rest 14\% (16\%), which are found exclusively in active regions of multipolar field configuration. Moreover, $n$ at the saddle bottom is significantly smaller in confined flares than that in eruptive ones. These results highlight the critical role of background field in regulating the eruptive behavior of two-ribbon flares.

Accept for publication in Astrophysical Journal Letters

References in corpus (8)

• Torus instability
• The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: SHARPs -- Space-weather HMI Active Region Patches
• Why Is the Great Solar Active Region 12192 Flare-Rich But CME-Poor?
• Numerical simulations of fast and slow coronal mass ejections
• Sigmoid-to-Flux-Rope Transition Leading to A Loop-Like Coronal Mass Ejection
• Catastrophe versus instability for the eruption of a toroidal solar magnetic flux rope
• Why is a flare-rich active region CME-poor?
• Investigating Energetic X-Shaped Flares on the Outskirts of A Solar Active Region

Cited by in corpus (2)

• Flare-productive active regions
• Why torus-unstable solar filaments experience failed eruption?