Kink instability of triangular jets in the solar atmosphere

Abstract

Context. It is known that hydrodynamic triangular jets (i.e. the jet with maximal velocity at its axis, which linearly decreases at both sides) are unstable to anti-symmetric kink perturbations. The inclusion of the magnetic field may lead to the stabilisation of the jets. Jets and complex magnetic fields are ubiquitous in the solar atmosphere, which suggests the possibility of the kink instability in certain cases. Aims. The aim of the paper is to study the kink instability of triangular jets sandwiched between magnetic tubes (or slabs) and its possible connection to observed properties of the jets in the solar atmosphere. Methods. A dispersion equation governing the kink perturbations is obtained through matching of analytical solutions at the jet boundaries. The equation is solved analytically and numerically for different parameters of jets and surrounding plasma. The analytical solution is accompanied by a numerical simulation of fully non-linear magnetohydrodynamic (MHD) equations for a particular situation of solar type II spicules. Results. Magnetohydrodynamic triangular jets are unstable to the dynamic kink instability depending on the Alfvén Mach number (the ratio of flow to Alfvén speeds) and the ratio of internal and external densities. When the jet has the same density as the surrounding plasma, only super-Alfvénic flows are unstable. However, denser jets are also unstable in a sub-Alfvénic regime. Jets with an angle to the ambient magnetic field have much lower thresholds of instability than field-aligned flows. Growth times of the kink instability are estimated to be 6−15 min for type I spicules and 5−60 s for type II spicules matching with their observed lifetimes. The numerical simulation of full non-linear equations shows that the transverse kink pulse locally destroys the jet in less than a minute in type II spicule conditions. Conclusions. Dynamic kink instability may lead to the full breakdown of MHD flows and consequently to an observed disappearance of spicules.

Origin of Quasi-periodic Pulsation at the Base of a Kink-unstable Jet

“…Kink instability is one of the prominent mechanisms to trigger the solar jets (see the review within Raouafi et al 2016). The observational and theoretical studies suggest that the rotational/twisting motions of the solar jets are directly linked with the helical kink instability (Shibata & Uchida 1986;Pariat et al 2009;Liu et al 2019;Zaqarashvili et al 2021).…”

MHD Wave Propagation and the Kelvin–Helmholtz Instability in an Asymmetric Magnetic Slab System

“…Other types of instabilities include the resonant flow instability (Tirry et al 1998;Taroyan & Erdélyi 2002) and shear flow instabilities (Taroyan & Ruderman 2011). Most recently, Zaqarashvili et al (2021) found that the dynamic kink instability of triangular jets can cause a complete breakdown of the MHD flows, which could result in the disappearance of spicules in solar atmosphere.…”

Magnetohydrodynamic Waves in an Asymmetric Magnetic Slab with Different External Flows

“…Further, the study of Barbulescu & Erdélyi (2018) can be considered a natural extension of this, as they focused on the effects of adding a steady background flow to the slab in an asymmetric slab system. A different type of extension is provided by Zaqarashvili et al (2021), who analyzed the stability of solar jets in slab geometries subject to a kink oscillation. This last example ties into another important line of investigation when it comes to generalizing slab models, especially the ones under the effect of background flows, namely, that of introducing nonuniformity in some parameters.…”