Filament formation in wind-cloud interactions. I. Spherical clouds in
  uniform magnetic fields
  Filamentary structures are ubiquitous in the interstellar medium, yet their
formation, internal structure, and longevity have not been studied in detail.
We report the results from a comprehensive numerical study that investigates
the characteristics, formation, and evolution of filaments arising from
magnetohydrodynamic interactions between supersonic winds and dense clouds.
Here we improve on previous simulations by utilising sharper density contrasts
and higher numerical resolutions. By following multiple density tracers, we
find that material in the envelopes of the clouds is removed and deposited
downstream to form filamentary tails, while the cores of the clouds serve as
footpoints and late-stage outer layers of these tails. Aspect ratios >12,
subsonic velocity dispersions ~0.1-0.3 of the wind sound speed, and magnetic
field amplifications ~100 are found to be characteristic of these filaments. We
also report the effects of different magnetic field strengths and orientations.
The magnetic field strength regulates vorticity production: sinuous filamentary
towers arise in non-magnetic environments, while strong magnetic fields inhibit
small-scale perturbations at boundary layers making tails less turbulent.
Magnetic field components aligned with the direction of the flow favour the
formation of pressure-confined flux ropes inside the tails, whilst transverse
components tend to form current sheets. Softening the equation of state to
nearly isothermal leads to suppression of dynamical instabilities and further
collimation of the tail. Towards the final stages of the evolution, we find
that small cloudlets and distorted filaments survive the break-up of the clouds
and become entrained in the winds, reaching velocities ~0.1 of the wind speed.
