  We present spatially resolved Herschel/PACS observations of the Orion Bar. We
have characterise the emission of the far-infrared fine-structure lines of
[CII] (158um), [OI] (63 and 145um), and [NII] (122um) that trace the gas local
conditions. The observed distribution and variation of the lines are discussed
in relation to the underlying geometry and linked to the energetics associated
with the Trapezium stars. These observations enable us to map the spatial
distribution of these fine-structure lines with a spatial resolution between 4"
and 11" and covering a total square area of about 120"x105". The spatial
profile of the emission lines are modelled using the radiative transfer code
Cloudy. We find that the spatial distribution of the [CII] line coincides with
that of the [OI] lines. The [NII] line peaks closer to the ionising star than
the other three lines, but with a small region of overlap. We can distinguish
several knots of enhanced emission within the Bar indicating the presence of an
inhomogenous and structured medium. The emission profiles cannot be reproduced
by a single photo-dissociation region, clearly indicating that, besides the
Bar, there is a significant contribution from additional photo-dissociation
region(s) over the area studied. The combination of both the [NII] and [OI]
145um lines can be used to estimate the [CII] emission and distinguish between
its ionised or neutral origin. We have calculated how much [CII] emission comes
from the neutral and ionised region, and find that at least 82% originates from
the photo-dissocciation region. Together, the [CII] 158um and [OI] 63 and 145um
lines account for 90% of the power emitted by the main cooling lines in the Bar
(including CO, H2, etc...), with [OI] 63um alone accounting for 72% of the
total.
