  It is known that hydrodynamic triangular jets are unstable to antisymmetric
kink perturbations. The inclusion of magnetic field may lead to the
stabilisation of the jets. Jets and complex magnetic fields are ubiquitous in
the solar atmosphere, which suggests the possibility of the kink instability in
certain cases. The aim of the paper is to study the kink instability of
triangular jets sandwiched between magnetic tubes/slabs and its possible
connection to observed properties of the jets in the solar atmosphere. A
dispersion equation governing the kink perturbations is obtained through
matching of analytical solutions at the jet boundaries. The equation is solved
analytically and numerically for different parameters of jets and surrounding
plasma. The analytical solution is accompanied by a numerical simulation of
fully nonlinear MHD equations for a particular situation of solar type II
spicules. MHD triangular jets are unstable to the dynamic kink instability
depending on the Alfven Mach number (the ratio of flow to Alfven speeds) and
the ratio of internal and external densities. When the jet has the same density
as the surrounding plasma, then only super Alfvenic flows are unstable.
However, denser jets are unstable also in sub Alfvenic regime. Jets with an
angle to the ambient magnetic field have much lower thresholds of instability
than field-aligned flows. Growth times of the kink instability are estimated as
6-15 min for type I spicules and 5-60 s for type II spicules matching with
their observed life times. Numerical simulation of full nonlinear equations
shows that the transverse kink pulse locally destroys the jet in less than a
minute in the conditions of type II spicules. Dynamic kink instability may lead
to full breakdown of MHD flows and consequently to observed disappearance of
spicules in the solar atmosphere.
