Dynamic Kink Instability and Transverse Motions of Solar Spicules

Hydrodynamic jets are unstable to the kink instability (m = 1 mode in cylindrical geometry) owing to the centripetal force, which increases the transverse displacement of the jet. When the jet moves along a magnetic field, the Lorentz force tries to decrease the displacement and stabilize the instability of sub-Alfvénic flows. The threshold of the instability depends on the Alfvén Mach number (the ratio of Alfvén and jet speeds). We suggest that the dynamic kink instability may be important to explain observed transverse motions of type II spicules in the solar atmosphere. We show that the instability may begin for spicules that rise up at the peripheries of vertically expanding magnetic flux tubes because of the decrease of the Alfvén speed in both the vertical and the radial directions. Therefore, inclined spicules may be more unstable and have higher transverse speeds. Periods and growth times of unstable modes in the conditions of type II spicules have values of 30 s and 25–100 s, respectively, which are comparable to the lifetime of the structures. This may indicate an interconnection between high-speed flow and the rapid disappearance of type II spicules in chromospheric spectral lines.