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 120x105. 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.
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