  The early B star S1 in the Rho Ophiuchus cloud excites an HII region and
illuminates a large egg-shaped photodissociation (PDR) cavity. The PDR is
restricted to the west and south-west by the dense molecular Rho Oph A ridge,
expanding more freely into the diffuse low density cloud to the north-east. We
analyze new SOFIA GREAT, GMRT and APEX data together with archival data from
Herschel/PACS, JCMT/HARPS to study the properties of the photo-irradiated
ionized and neutral gas in this region. The tracers include [C II] at 158
micron, [O I] at 63 and 145 micron, J=6-5 transitions of CO and 13CO, HCO+
(4-3), radio continuum at 610 and 1420 MHz and HI at 21 cm. The PDR emission is
strongly red-shifted to the south-east of the nebula, and primarily
blue-shifted on the north western side. The [C II] and and [O I]63 spectra are
strongly self-absorbed over most of the PDR. By using the optically thin
counterparts, [13C II] and [O I]145 respectively, we conclude that the
self-absorption is dominated by the warm (>80 K) foreground PDR gas and not by
the surrounding cold molecular cloud. We estimate the column densities of C+
and O of the PDR to be 3e18 and 2e19 cm^-2, respectively. Comparison of stellar
far-ultraviolet flux and reprocessed infrared radiation suggest enhanced
clumpiness of the gas to the north-west. Analysis of the emission from the PDR
gas suggests the presence of at least three density components consisting of
high density (10^6 cm^-3) clumps, medium density (10^4 cm^-3) and diffuse (10^3
cm^-3) interclump medium. The medium density component primarily contributes to
the thermal pressure of the PDR gas which is in pressure equilibrium with the
molecular cloud to the west. We find that the PDR is tilted and warped with the
south-eastern side of the cavity being denser on the front and the
north-western side being denser on the rear.
