We report the first detection of the 205 µm 3 P 1 → 3 P 0 [NII] line from a ground-based observatory using a direct detection spectrometer. The line was detected from the Carina star formation region using the South Pole Imaging Fabry-Perot Interferometer (SPIFI) on the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) at South Pole. The [NII] 205 µm line strength indicates a low-density (n ∼ 32 cm −3 ) ionized medium, similar to the low-density ionized halo reported previously in its [OIII] 52 and 88 µm line emission. When compared with the ISO [CII] observations of this region, we find that 27% of the [CII] line emission arises from this low-density ionized gas, but the large majority (∼ 73%) of the observed [CII] line emission arises from the neutral interstellar medium. This result supports and underpins prior conclusions that most of the observed [CII] 158 µm line emission from Galactic and extragalactic sources arises from the warm, dense photodissociated surfaces of molecular clouds. The detection of the [NII] line demonstrates the utility of Antarctic sites for THz spectroscopy.

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Spatial-spatial ('; b) integrated intensity maps for the three transitions observed with ASTRO (top three panels) and, for comparison, the three transitions observed at the BL 7 m (bottom three panels). Transitions are identified at left on each panel. The emission is integrated over all velocities where data are available. These values of (v min , v max ) are: [C i ], (À90, 150); CO(7-6), (À30, 120); CO(4-3), (À150, 150); CO(1-0), (À150, 150); 13 CO(1-0), (À150, 150); CS(2-1), (À150, 150). All six maps have been smoothed to the same 2 0 resolution. The scale on the color bars have been multiplied by 2 to correct for an error in the plotting routine used in the published version of the maps. 395

We present narrow-bandwidth submillimeter-wave sky opacity measurements made from the South Pole between 1995 February 9 and November 17, a period that includes an entire Austral winter. These measurements were made with the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) using a heterodyne receiver tuned to a band that includes the 492 GHz Ðne-structure line of neutral atomic carbon. The zenith optical depth was below 0.72 half the time during the Austral winter and spring, and it reached values as low as 0.34 on day 232. The stability was also remarkably good : the opacity remained below 1.0 for weeks at a time. The South Pole is therefore an excellent site for submillimeter astronomy throughout the Austral winter and spring. The functional relationship between 492 GHz opacity and measured precipitable water vapor shows that a signiÐcant fraction of the opacity is caused by atmospheric constituents other than water vapor, indicating the need for accurate, sitedependent atmospheric modeling when opacity measurements at lower frequencies are extrapolated into the submillimeter.