High-resolution H I observations of H II regions. III. Photodissociation regions and the magnetic field near orion B

H I observations of the H II region Orion B are presented with an angular resolution of 50″ (0.1 pc) and velocity resolution of 0.64 km s^-1. Three kinematically distinct H I layers are detected in absorption. It is argued that one of these H I components originates in a photodissociation region (PDR) directly outside the ionization front in front of the H II region. In addition to the H I absorption, H I emission is detected, most likely originating in a PDR behind the H II region. Combining the H I emission and absorption data yields the conclusion that a range of temperatures exists in the PDRs, a conclusion corroborated by existing [C II] 158 μm line measurements of the PDRs. While the H I absorption data show the presence of gas colder than ∼20 K (which is too cold to contribute significantly to the [C II] emission), the H I and [C II] data can be brought in agreement if the emitting H I is a temperature of at least several hundred degrees. These results are interpreted in terms of a clumpy PDR model, where the colder gas is located in dense clumps, which are embedded in a warmer, low-density interclump medium. An analysis of the Zeeman effect as determined from the observed circular polarization yields line-of-sight magnetic fields of 28 and 63 μG for two of the H I layers, at a reduced angular resolution of 100″. It is concluded that the magnetic field strength and the total pressure in the dense PDR are higher than in the more tenuous cloud envelope. However, in the cloud envelope the pressure is dominated by the magnetic field, while in the PDR turbulent pressure dominates.