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We present high resolution [NII] 205 micrometer ^3P_1-^3P_0 spectra obtainedwith Herschel-HIFI towards a small sample of far-infrared bright star formingregions in the Galactic plane: W31C (G10.6-0.4), W49N (G43.2-0.1), W51(G49.5-0.4), and G34.3+0.1. All sources display an emission line profileassociated directly with the HII regions themselves. For the first time we alsodetect absorption of the [NII] 205 micrometer line by extended low-densityforeground material towards W31C and W49N over a wide range of velocities. Weattribute this absorption to the warm ionised medium (WIM) and findN(N^+)\approx 1.5x10^17 cm^-2 towards both sources. This is in agreement withrecent Herschel-HIFI observations of [CII] 158 micrometer, also observed inabsorption in the same sight-lines, if \approx7-10 % of all C^+ ions exist inthe WIM on average. Using an abundance ratio of [N]/[H] = 6.76x10^-5 in the gasphase we find that the mean electron and proton volume densities are ~0.1-0.3cm^-3 assuming a WIM volume filling fraction of 0.1-0.4 with a correspondingline-of-sight filling fraction of 0.46-0.74. A low density and a high WIMfilling fraction are also supported by RADEX modelling of the [NII] 205micrometer absorption and emission together with visible emission linesattributed mainly to the WIM. The detection of the 205 micrometer line inabsorption emphasises the importance of a high spectral resolution, and alsooffers a new tool for investigation of the WIM.

First detection of [N II] 205 micrometer absorption in interstellar gas