CARA Science: Submillimeter Site Characterization

For more information, see the site testing section of the Stark et al. decadal report and Lane 1998

While the altitude of South Pole Station (2835 m) is lower than that of some other existing (e.g., Mauna Kea, Hawaii) or proposed (e.g., Atacama desert, Chile) millimeter/submillimeter observing sites---resulting in higher atmospheric pressure and hence broader telluric lines---the polar atmosphere is significantly drier than anywhere else on Earth.

At millimeter and submillimeter wavelengths, atmospheric transmission is superior to that at other sites, and stability is excellent (Chamberlin & Bally 1994; Chamberlin & Bally 1995; Chamberlin et al. 1997).

The South Pole is the best observatory site on Earth for submillimeter and millimeter astronomy. Skydip measurements taken at 492 GHz with the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) throughout 1995 have provided the first systematic, long-term measurements of submillimeter-wave atmospheric transparency at an astronomical site.

Profiles of temperature, pressure, and water vapor above the Pole, measured by balloon-borne radiosonde at least once a day for several decades (Schwerdtfeger 1984), consistently show precipitable water vapor (PWV) values which are near saturation for air at the observed temperature and pressure. The PWV values which are consistent with saturation are extremely low because the polar air is dessicated by the frigid temperature.

Saturation PWV at Pole, 1961-1998. Quartiles of saturation-point PWV distribution during the winter (day-of-year 100-300) for 1961-1998, calculated from balloon-borne pressure and temperature measurements. This figure illustrates the long-term stability of the South Pole climate.

CARA experiments have directly measured both millimeter and submillimeter-wave atmospheric opacity at the South Pole using skydip techniques. Over 1100 skydip observations were made at 492 GHz (609 microns) with the 1.7 m AST/RO telescope during the 1995 observing season (Chamberlin 1997). The zenith opacity derived by fitting a single-slab atmospheric model to the data is shown below as a function of day number in 1995.

South Pole 492 GHz. Opacity during 1995. Shown is the measured 492 GHz zenith opacity, tau-nought, at the South Pole plotted as a function of days elapsed since 1 Jan 1995. Throughout this period, the precipitable water vapro (PWV) was directly measured by balloon-borne radiosondes and LIDAR. There is a strong correlation between PWV and tau. For the 1995 winter season, the quartiles of the 492 GHz zenith opacity cumulative distribution function are (25%:50%:75%) = (0.55:0.70:0.81). The minimum 492 GHz opacity at Pole is limited to 0.33+/-0.02 largely by the wings of the oxygen line at 487 GHz. (Chamberlin 1997).

Radiosonde measurements made twice daily in summer and once daily in winter by the South Pole meteorology office have been used to derive values for the column of precipitable water vapor above the South Pole (Chamberlin 1995, 1997).

Quartile values from the PWV cumulative distribution function and comparisons with corresponding values for Mauna Kea and for Atacama are listed in the table below and plotted in the accompanying figure. For each site, the data are separated into the best 6-month period and the remainder of the year. Of the three sites, South Pole has by far the lowest PWV, during Austral summer as well as winter. In fact, the 75th percentile value for the wetter half of the year at the South Pole is less than the 25th percentile value for the drier half of the year at Mauna Kea or Atacama.

The South Pole is a considerably drier site than Mauna Kea or Atacama. During the wettest quartile at the South Pole, the total precipitable water vapor is lower than during the driest quartile at either Mauna Kea or Atacama.

For more information, see Lane, A. P. 1998, "Submillimeter Transmission at South Pole", in Astrophsyics From Antarctica, A.S.P. Conf. Ser., eds. G. Novak and R. H. Landsberg, (San Francisco, CA: Astr. Soc. of the Pacific).

Back to the main site testing page.