Parametric Study of the Torus Instability Threshold
Abstract
The torus instability of an arched current channel has been suggested to initiate and drive major solar and stellar eruptions. Its threshold, given by the critical decay index of the equilibrium external poloidal field (the so-called strapping field) at the position of the current channel, is insufficiently known. Here, we carry out a parametric numerical study of the threshold, employing the force-free Titov-Démoulin (TD) equilibrium of a line-tied partial toroidal current channel and flux rope. This addresses the scatter of the threshold about its canonical value, ncr=3/2. Values scattering in the range ncr≈\,1--2 are typically found in numerical and observational studies of flux rope eruptions on the Sun. For zero external toroidal (guide, or shear) field and approximately semicircular geometry (corresponding to minimal photospheric line-tying), we find the threshold to lie in the theoretically expected range of ≈\,1--1.5. An external toroidal field introduces a strong stabilizing effect on the instability, raising the threshold up to ∼\,2.5, which can explain observational and numerical results above the canonical value. Line-tying is found to act as a stabilizer as well. We also consider the approximate threshold based on the potential field and find a very good agreement with the exact numerical value, provided the horizontal component perpendicular to the flux rope axis is used to approximate the external poloidal field.
Versions
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BibTeX
@misc{chen2026parametric,
title = {{Parametric Study of the Torus Instability Threshold}},
author = {Jun Chen and Bernhard Kliem and Rui Liu},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2608.10208},
eprint = {2608.10208},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.10208},
note = {CC BY-NC-SA 4.0},
}
RIS
TY - UNPB
TI - Parametric Study of the Torus Instability Threshold
AU - Jun Chen
AU - Bernhard Kliem
AU - Rui Liu
PY - 2026
DA - 2026/09/22
PB - arXiv
DO - 10.48550/arxiv.2608.10208
UR - https://arxiv.org/abs/2608.10208
AB - The torus instability of an arched current channel has been suggested to initiate and drive major solar and stellar eruptions. Its threshold, given by the critical decay index of the equilibrium external poloidal field (the so-called strapping field) at the position of the current channel, is insufficiently known. Here, we carry out a parametric numerical study of the threshold, employing the force-free Titov-Démoulin (TD) equilibrium of a line-tied partial toroidal current channel and flux rope. This addresses the scatter of the threshold about its canonical value, n_cr=3/2. Values scattering in the range n_cr≈\,1--2 are typically found in numerical and observational studies of flux rope eruptions on the Sun. For zero external toroidal (guide, or shear) field and approximately semicircular geometry (corresponding to minimal photospheric line-tying), we find the threshold to lie in the theoretically expected range of ≈\,1--1.5. An external toroidal field introduces a strong stabilizing effect on the instability, raising the threshold up to ∼\,2.5, which can explain observational and numerical results above the canonical value. Line-tying is found to act as a stabilizer as well. We also consider the approximate threshold based on the potential field and find a very good agreement with the exact numerical value, provided the horizontal component perpendicular to the flux rope axis is used to approximate the external poloidal field.
ER -