Unstable drift mode driven by shear plasma flow in solar spicules

Abstract

Context. The lower solar atmosphere contains at any moment a large number of spicules comprising plasma that moves towards the upper layers with typical axial velocities of 20−30 km s −1 . It is expected that these flows as well as the plasma density are inhomogeneous in the perpendicular direction. The presence of such a density gradient implies the existence of drift waves, while the inhomogeneity of the flow velocity can cause the growth of such modes. Aims. The stability of the drift waves will be discussed within the two-fluid theory taking into account the ion temperature and the stress tensor effects. Methods. An analytical linear normal mode analysis is used within the local approximation. Results. A detailed derivation of the hot ion contribution is performed. A dispersion equation is derived and the stability/instability conditions are discussed in detail for the parameter range appropriate for solar spicules. The drift mode appears to be highly unstable for typical spicule characteristic lengths of the density and the shear flow gradients, i.e. in the range of a few hundred meters up to a few kilometers, yielding wave frequencies of the order of a few Hz. Conclusions. Hence, the waves and the instabilities develop at reasonable time scales regarding the life times of spicules that are measured in minutes.