Laboratory study of the torus instability threshold in solar-relevant, line-tied magnetic flux ropes

Abstract

Coronal mass ejections (CME) occur when long-lived magnetic flux ropes (MFR) anchored to the solar surface destabilize and erupt away from the Sun. This destabilization is often described in terms of an ideal magnetohydrodynamic (MHD) instability called the torus instability. It occurs when the external magnetic field decreases sufficiently fast such that its decay index, $n=-z\partial (\ln B)/\partial z$ is larger than a critical value, $n>n_{\mathrm{cr}}$, where $n_{\mathrm{cr}}=1.5$ for a full, large aspect ratio torus. However, when this is applied to solar MFRs, a range of conflicting values for $n_{\mathrm{cr}}$ is found in the literature. To investigate this discrepancy, we have conducted laboratory experiments on arched, line-tied flux ropes and have applied a theoretical model of the torus instability. Our model describes an MFR as a partial torus with footpoints anchored in a conducting surface and numerically calculates various magnetic forces on it. This calculation yields a better prediction of $n_{\mathrm{cr}}$ which takes into account the specific parameters of the MFR. We describe a systematic methodology to properly translate laboratory results to their solar counterparts, provided that the MFRs have sufficiently small edge safety factor, or equivalently, large enough twist. After this translation, our model predicts that $n_{\mathrm{cr}}$ in solar conditions often falls near $n_{\mathrm{cr}}^{\mathrm{Sol}}\sim 0.9$ and within a larger range of $n_{\mathrm{cr}}^{\mathrm{Sol}}\sim (0.7,1.2)$ depending on the parameters. The methodology of translating laboratory MFRs to their solar counterparts enables quantitative investigations of the initiation of CMEs through laboratory experiments. These experiments allow for new physics insights that are required for better predictions of space weather events but are difficult to obtain otherwise.

Submitted to the Astrophysical Journal

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Cited by in corpus (9)

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• Where and how does a decay-index profile become saddle-like?
• Three-dimensional velocity fields of the solar filament eruptions detected by CHASE
• Stability of the coronal magnetic field around large confined and eruptive solar flares
• The Initiation Mechanism of the First On-disk X-Class Flare of Solar Cycle 25
• A Data-constrained Magnetohydrodynamic Simulation of Successive X-class Flares in Solar Active Region 13842 I. Dynamics of the Solar Eruption Associated with the X7.1 Solar Flare
• The Kinematical Behavior of Solar Eruptive Filaments Affected by the Poloidal Magnetic Field
• Laboratory study of the failed torus mechanism in arched, line-tied, magnetic flux ropes