  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,
then the Lorentz force tries to decrease the displacement and stabilises the
instability of sub-Alfvenic flows. The threshold of the instability depends on
the Alfven Mach number (the ratio of Alfven and jet speeds). We suggest that
the dynamic kink instability may be of importance to explain observed
transverse motions of type II spicules in the solar atmosphere. We show that
the instability may start for spicules which rise up at the peripheries of
vertically expanding magnetic flux tubes owing to the decrease of the Alfven
speed in both, the vertical and the radial directions. Therefore, inclined
spicules may be more unstable and have more higher transverse speeds. Periods
and growth times of unstable modes in the conditions of type II spicules have
the values of 30 s and 25-100 s, respectively, which are comparable to the life
time of the structures. This may indicate to the interconnection between high
speed flow and rapid disappearance of type II spicules in chromospheric
spectral lines.
