  We have used the AMR hydrodynamic code, MG, to perform idealised 3D MHD
simulations of the formation of clumpy and filamentary structure in a thermally
unstable medium without turbulence. A stationary thermally unstable spherical
diffuse atomic cloud with uniform density in pressure equilibrium with low
density surroundings was seeded with random density variations and allowed to
evolve. A range of magnetic field strengths threading the cloud have been
explored, from beta=0.1 to beta=1.0 to the zero magnetic field case
(beta=infinity), where beta is the ratio of thermal pressure to magnetic
pressure. Once the density inhomogeneities had developed to the point where
gravity started to become important, self-gravity was introduced to the
simulation. With no magnetic field, clouds and clumps form within the cloud
with aspect ratios of around unity, whereas in the presence of a relatively
strong field (beta=0.1) these become filaments, then evolve into interconnected
corrugated sheets that are predominantly perpendicular to the magnetic field.
With magnetic and thermal pressure equality (beta=1.0), filaments, clouds and
clumps are formed. At any particular instant, the projection of the 3D
structure onto a plane parallel to the magnetic field, i.e. a line of sight
perpendicular to the magnetic field, resembles the appearance of filamentary
molecular clouds. The filament densities, widths, velocity dispersions and
temperatures resemble those observed in molecular clouds. In contrast, in the
strong field case beta=0.1, projection of the 3D structure along a line of
sight parallel to the magnetic field reveals a remarkably uniform structure.
