First measurements of the infrared sky brightness at Dome C, Antarctica

Dome C, Antarctica, (75 degrees south, 123 degrees east, 3250 m) is one of the coldest and driest locations on Earth, with exceptionally low winds throughout the atmosphere. It therefore has the potential to be an ideal site for astronomical observations. It is also an excellent site for the validation of satellite instruments. A Fourier transform infrared interferometer was deployed at Dome C during two austral summer seasons ( 2003 January and 2003 December/2004 January) for the purpose of acquiring satellite validation data. However, these data are also useful for understanding the infrared characteristics of the atmosphere for future astronomical experiments at Dome C. The Polar Atmospheric Emitted Radiance Interferometer measured the downwelling infrared radiance from the atmosphere ( sky brightness) from 3 to 20 mm. Over 100 radiosondes were also launched during this time period. Typical measured values of the sky brightness in the clearest portions of the M, N, and Q bands are 0.9, 43, and 310 Jy arcsec(-2), respectively. The lowest measured values of sky brightness within these bands are 0.4, 34, and 200 Jy arcsec(-2). The spectral region of the Q band from about 18.7 to 19 mu m is expected to be an excellent window for observations made from the Antarctic Plateau. The sky brightness has been measured between 10.60 and 11.30 mu m in the N band for comparisons to earlier studies at South Pole Station; the values in this band are similar to those in the 8.20 - 8.40 mu m band. For the period of time covered by our observations, the sky brightness in the dark portions of the N band was less than about 50 - 60 Jy arcsec(-2) for about 10% of the time, and less than about 75 Jy arcsec(-2) for about 50% of the time. During a 5 day period of clear skies, the mean sky brightness was 47.7 Jy arcsec(-2), with a variation about this mean of 4.4 Jy arcsec(-2) ( 1 sigma). Calculations of the summertime atmospheric transmission under clear skies over Dome C show that portions of the M, N, and Q bands have transmission greater than 95%, with some spectral regions greater than 99%.