Magnetic fields, stellar feedback, and the geometry of H II Regions
  Magnetic pressure has long been known to dominate over gas pressure in atomic
and molecular regions of the interstellar medium. Here I review several recent
observational studies of the relationships between the H^+, H^0 and H_2 regions
in M42 (the Orion complex) and M17. A simple picture results. When stars form
they push back surrounding material, mainly through the outward momentum of
starlight acting on grains, and field lines are dragged with the gas due to
flux freezing. The magnetic field is compressed and the magnetic pressure
increases until it is able to resist further expansion and the system comes
into approximate magnetostatic equilibrium. Magnetic field lines can be
preferentially aligned perpendicular to the long axis of quiescent cloud before
stars form. After star formation and pushback occurs ionized gas will be
constrained to flow along field lines and escape from the system along
directions perpendicular to the long axis. The magnetic field may play other
roles in the physics of the H II region and associated PDR. Cosmic rays may be
enhanced along with the field and provide additional heating of atomic and
molecular material. Wave motions may be associated with the field and
contribute a component of turbulence to observed line profiles.
