Onset of the Kelvin-Helmholtz instability in partially ionized magnetic flux tubes

TL;DR

This paper investigates how partial ionization affects the Kelvin-Helmholtz instability in solar prominence flux tubes, revealing that neutrals can trigger KHI at sub-Alfvénic flows, with collisions reducing but not eliminating the instability.

Contribution

It provides a linear analysis of KHI onset in partially ionized magnetic flux tubes, including an analytical approximation of growth rates and application to solar prominences.

Findings

Neutral components can trigger KHI at sub-Alfvénic flows.

Ion-neutral collisions reduce but do not suppress KHI growth.

Turbulent flows in prominences may result from KHI in partially ionized plasmas.

Abstract

Context. Recent observations of solar prominences show the presence of turbulent flows that may be caused by Kelvin-Helmholtz instabilites (KHI). However, the observed flow velocities are below the classical threshold for the onset of KHI in fully ionized plasmas. Aims. We investigate the effect of partial ionization on the onset of KHI in dense and cool cylindrical magnetic flux tubes surrounded by a hotter and lighter environment. Methods. The linearized governing equations of a partially ionized two-fluid plasma are used to describe the behavior of small-amplitude perturbations superimposed on a magnetic tube with longitudinal mass flow. A normal mode analysis is performed to obtain the dispersion relation for linear incompressible waves. We focus on the appearance of unstable solutions and study the dependence of their growth rates on various physical parameters. An analytical approximation of the KHI linear growth rate for slow flows and strong ion-neutral coupling is obtained. An application to solar prominence threads is given. Results. The presence of a neutral component in a plasma may contribute to the onset of the KHI even for sub-Alfv\'enic longitudinal shear flows. Collisions between ions and neutrals reduce the growth rates of the unstable perturbations but cannot completely suppress the instability. Conclusions. Turbulent flows in solar prominences with sub-Alfv\'enic flow velocities may be interpreted as consequences of KHI in partially ionized plasmas.