The magnetic field structure in molecular cloud filaments
  We explore the structure of magnetic field lines in and around filaments in
simulations of molecular clouds undergoing global, multi-scale gravitational
collapse. In these simulations, filaments are not in a static equilibrium, but
are long-lived flow structures that accrete gas from their environment and
direct it toward clumps embedded in the filament or at the nodes at the
conjunction with other filaments. In this context, the magnetic field is
dragged by the collapsing gas, so its structure must reflect the flow that
generates the filament. Around the filament, the gas is accreted onto it, and
the magnetic lines must then be perpendicular to the filament. As the gas
density increases, the gas flow changes direction, becoming almost parallel to
the filament, and magnetic lines also tend to align with it. At the spine of
the filament, however, magnetic lines become perpendicular again since they
must connect to lines on the opposite side of the filament, resulting in
"U"-shaped magnetic structures, which tend to be stretched by the longitudinal
flow along the filament. Magnetic diffusive processes, however, allow the gas
to continue to flow. Assuming a stationary state in which the ram pressure of
the flow balances the magnetic tension, the curvature of the field lines is
determined by the diffusion rate. We derive an expression relating the
curvature of the field lines to the diffusive coefficient, which may be used to
observationally determine the nature of the diffusive process.
