Flute and kink instabilities in a dynamically twisted flux tube with anisotropic plasma viscosity

Abstract

Magnetic flux tubes such as those in the solar corona are subject to a number of instabilities. Important among them is the kink instability which plays a central part in the nanoflare theory of coronal heating, and for this reason in numerical simulations it is usually induced by tightly-controlled perturbations and studied in isolation. In contrast, we find that fluting modes of instability are readily excited when disturbances are introduced in our magnetohydrodynamic flux tube simulations by dynamic twisting of the flow at the boundaries. We also find that the flute instability, which has been theorised but rarely observed in the coronal context, is strongly enhanced when plasma viscosity is assumed anisotropic. We proceed to investigate the co-existence and competition between flute and kink instabilities for a range of values of the resistivity and of the parameters of the anisotropic and isotropic models of viscosity. We conclude that while the flute instability cannot prevent the kink from ultimately dominating, it can significantly delay its development especially at strong viscous anisotropy induced by intense magnetic fields.

Safety First: Stability and Dissipation of Line-tied Force-free Flux Tubes in Magnetized Coronae

“…In contrast to Quinn & Simitev (2022) we do not find a significantly delayed onset of the kink mode due to the growth of m = 2 (fluting) patterns. This may be due to the absence of resistive/viscous effects in FFE, which effectively propagates all perturbations at the fastest velocity, the speed of light.…”

“…The possibility of mode mixing in flux tubes was discussed, though not observed, in the context of the solar corona by Quinn & Simitev (2022). Instead of perturbing an equilibrium configuration with a specific profile that seeds the growth of an m = 1 (kink) mode, Quinn & Simitev (2022) rely on noise introduced by the continuous twisting of a flux tube to drive the instability. With this strategy, which is somewhat closer to the realistic flux-tube evolution, they allow for the development of m > 1 modes that are usually not addressed in the literature.…”

“…Magnetospheric dissipation likely drives at least some of the abundant X-ray activity observed from compact object magnetospheres with active coronae, such as magnetars (e.g., Göğüş et al 1999Göğüş et al , 2000Göğüş et al , 2001Rea et al 2009;Rea & Esposito 2011;Kaspi & Beloborodov 2017;Esposito et al 2020) and magnetized black hole accretion disks (e.g., Haardt et al 1994;Di Matteo et al 1999;Chartas et al 2009;Uttley et al 2014;Wilkins & Gallo 2015). The stability and dynamics of flux bundles in (highly) magnetized environments are well studied and observed, including for applications to astrophysical jets (e.g., Lyubarskii 1999;Giannios & Spruit 2006;Lapenta et al 2006;Narayan et al 2009;Alves et al 2018;Bromberg et al 2019;Davelaar et al 2020) and the solar corona (e.g., Raadu 1972;Hood & Priest 1979, 1981Linton et al 1998;Lapenta et al 2006;Kumar & Cho 2014;Florido-Llinas et al 2020;Xu et al 2020;Quinn & Simitev 2022). Yet, insights from the considered systems, often periodic or highly constrained by the specific astrophysical scenario, cannot answer the most fundamental questions for highly magnetized flux tubes with field line footpoints frozen (line-tied) to a stellar or disk surface boundary at both ends: If, when, and how does a flux tube become unstable, and what is the amount of dissipated magnetic energy during its instability?…”