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

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 were used to describe the behavior of small-amplitude perturbations superimposed on a magnetic tube with longitudinal mass flow. A normal mode analysis was performed to obtain the dispersion relation for linear incompressible waves. We focused on the appearance of unstable solutions and studied the dependence of their growth rates on various physical parameters. We obtained an analytical approximation of the KHI linear growth rate for slow flows and strong ion-neutral coupling. We applied this to solar prominence threads. Results. The presence of a neutral component in a plasma may contribute to the onset of the KHI even for sub-Alfvenic 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-Alfvenic flow velocities may be interpreted as consequences of KHI in partially ionized plasmas.