Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Critical Height of the Torus Instability in Two-Ribbon Solar Flares   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.

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The categories of this paper are: q-alg
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The categories of this paper are: ao-sci
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The categories of this paper are: hep-th
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The categories of this paper are: cond-mat
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The categories of this paper are: patt-sol
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The categories of this paper are: nucl-ex
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The categories of this paper are: supr-con
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The categories of this paper are: math
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The categories of this paper are: q-bio
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The categories of this paper are: alg-geom
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The categories of this paper are: nucl-th
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The categories of this paper are: quant-ph
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The categories of this paper are: stat
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The categories of this paper are: plasm-ph
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The categories of this paper are: eess
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The categories of this paper are: acc-phys