  An hourglass-shaped magnetic field pattern arises naturally from the
gravitational collapse of a star-forming gas cloud. Most studies have focused
on the prestellar collapse phase, when the structure has a smooth and monotonic
radial profile. However, most observations target dense clouds that already
contain a central protostar, and possibly a circumstellar disk. We utilize an
analytic treatment of the magnetic field along with insights gained from
simulations to develop a more realistic magnetic field model for the
protostellar phase. Key elements of the model are a strong radial magnetic
field in the region of rapid collapse, an off-center peak in the magnetic field
strength (a consequence of magnetic field dissipation in the circumstellar
disk), and a strong toroidal field that is generated in the region of rapid
collapse and outflow generation. A model with a highly pinched and twisted
magnetic field pattern in the inner collapse zone facilitates the
interpretation of magnetic field patterns observed in protostellar clouds.
