Striations in molecular clouds: Streamers or MHD waves?
  Dust continuum and molecular observations of the low column density parts of
molecular clouds have revealed the presence of elongated structures which
appear to be well aligned with the magnetic field. These so-called striations
are usually assumed to be streams that flow towards or away from denser
regions. We perform ideal magnetohydrodynamic (MHD) simulations adopting four
models that could account for the formation of such structures. In the first
two models striations are created by velocity gradients between ambient,
parallel streamlines along magnetic field lines. In the third model striations
are formed as a result of a Kelvin-Helmholtz instability perpendicular to field
lines. Finally, in the fourth model striations are formed from the nonlinear
coupling of MHD waves due to density inhomogeneities. We assess the validity of
each scenario by comparing the results from our simulations with previous
observational studies and results obtained from the analysis of CO (J = 1 - 0)
observations from the Taurus molecular cloud. We find that the first three
models cannot reproduce the density contrast and the properties of the spatial
power spectrum of a perpendicular cut to the long axes of striations. We
conclude that the nonlinear coupling of MHD waves is the most probable
formation mechanism of striations.
