  Previous work has shown the Orion Bar to be an interface between ionized and
molecular gas, viewed roughly edge on, which is excited by the light from the
Trapezium cluster. Much of the emission from any star-forming region will
originate from such interfaces, so the Bar serves as a foundation test of any
emission model. Here we combine X-ray, optical, IR and radio data sets to
derive emission spectra along the transition from H+ to H0 to H2 regions. We
then reproduce the spectra of these layers with a simulation that
simultaneously accounts for the detailed microphysics of the gas, the grains,
and molecules, especially H2 and CO. The magnetic field, observed to be the
dominant pressure in another region of the Orion Nebula, is treated as a free
parameter, along with the density of cosmic rays. Our model successfully
accounts for the optical, IR and radio observations across the Bar by including
a significant magnetic pressure and also heating by an excess density of cosmic
rays, which we suggest is due to cosmic rays being trapped in the compressed
magnetic field. In the Orion Bar, as we had previously found in M17, momentum
carried by radiation and winds from the newly formed stars pushes back and
compresses the surrounding gas. There is a rough balance between outward
momentum in starlight and the total pressure in atomic and molecular gas
surrounding the H+ region. If the gas starts out with a weak magnetic field,
the starlight from a newly formed cluster will push back the gas and compress
the gas, magnetic field, and cosmic rays until magnetic pressure becomes an
important factor.
