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 = \infty ) , 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 .