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., eruptive and 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 crit corresponding to the theoretical threshold (n crit = 1.5) of the torus instability. We found that h crit is approximately half of the distance between the centroids of opposite polarities in active regions and that the distribution of h 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.

On the Factors Determining the Eruptive Character of Solar Flares

“…This is significantly different from the findings of Wang et al (2017), who reported these fractions as 85% and 15%, respectively, based on an event sample largely comprised of smaller flares (75% of their analyzed events were <M5.0).…”

“…Cheng et al 2011;Wang et al 2017). Wang et al (2017) suggested an empirical relation of the form h crit 0.5 · d PC (cf. their Fig.…”

“…As a reason for the slightly different result, we suspect the different flare sizes covered in our study. Wang et al (2017) analyzed 60 flares down to GOES class M1.0, where the majority of events (75%) were classified as <M5.0. In our study we considered exclusively large events, i.e., flares of GOES class M5.0 and larger.…”

“…Recently, Wang et al (2017) presented a statistical analysis of 60 flares. They found that in ARs that host eruptive flares, h crit is reached at significantly lower coronal heights.…”

Failed prominence eruptions near 24 cycle maximum

“…Moreover, the field strength for the eruptive flares was weaker than that for the confined events at the same heights. Wang et al (2017) found that the decay index increased monotonously with height in the majority (∼ 85%) of studied 60 flaring regions and reached the critical value for the torus instability (n c = 1.5) at significantly higher heights for confined flares than for eruptive ones. Saddle-like n-profiles in the other regions had significantly smaller values of n at the saddle bottom in confined flares than that in eruptive ones.…”