Abstract.A good astronomical site must fulfill several criteria including low atmospheric turbulence and low wind speeds. It is therefore important to have a detailed knowledge of the temperature and wind conditions of a location considered for future astronomical research. Antarctica has unique atmospheric conditions that have already been exploited at the South Pole station. Dome C, a site located on a local maximum of the Antarctic plateau, is likely to have even better conditions. In this paper we present the analysis of two decades of wind speed measurements taken at Dome C by an automated weather station (AWS). We also present temperature and wind speed profiles taken over four Antarctic summers using balloon-borne weather sondes. We will show that as well as having one of the lowest average wind speed ever recorded at an existing or potential observatory, Dome C also has an extremely stable upper atmosphere and a very low inversion layer.

During the austral winter 2005, the first astronomical site testing campaign were performed at Dome C, in Antarctica. Thirty-five meteorological balloons equipped with microthermal sensors were used to sense the vertical profile of the optical turbulence intensity C 2 N above Dome C up to 20 km. All the profiles of the 2005 campaign are statistically analyzed. We provide the median C 2N profiles and the mean potential temperature, mean horizontal wind speed, and mean direction profiles for the three seasons covered by this campaign (autumn, winter, and beginning of the spring). The structure of the optical turbulence in the atmosphere above Dome C is analyzed and compared with the well-known median C 2 N profiles of midlatitude sites. Of the whole optical turbulence, 80% lies within the first 33 m above the ground and 9% in the upper part of the boundary layer, between 33 m and 1 km above the ground. The remaining 11% are in the free atmosphere. This is an extreme situation when compared with "classical" midlatitude sites where the surface layer extends up to 200 m. This strong and thin surface layer is the result of the kinetic turbulent mixing of air combined with a strong potential temperature gradient. The site is characterized from the adaptive optics point of view. Seeing, isoplanatic angle, and coherence time are estimated for each considered seasons. A four-layer decomposition for each season is provided for adaptive optics simulations. For high angular astronomy, a telescope at Dome C needs to be elevated over this surface layer, or a specific GLAO needs to be designed. Combined with the unique possibility of performing continuous observation from Antarctica, scientific programs such as microlensing, pulsating stars, and asteroseismology become feasible.

We report site testing results obtained in night-time during the polar autumn and winter at Dome C. These results were collected during the first Concordia winterover by A. Agabi. They are based upon seeing and isoplanatic angle monitoring, as well as in-situ balloon measurements of the refractive index structure constant profiles C 2 n (h). Atmosphere is divided into two regions: (i) a 36 m high surface layer responsible of 87% of the turbulence and (ii) a very stable free atmosphere above with a median seeing of 0.36±0.19 arcsec at an elevation of h = 30 m. The median seeing measured with a DIMM placed on top of a 8.5 m high tower is 1.3±0.8 arcsec.

Abstract.Results from the 1995 season of site-testing experiments at the South Pole are presented, in which the seeing was measured using balloon-borne microthermal probes. Our analysis shows a marked division of the atmosphere into two characteristic regions: (i) a highly turbulent boundary layer (0 − 220 m) associated with a strong temperature inversion and wind shear, and (ii) a very stable free atmosphere. The mean seeing, averaged over 15 balloon flights, was measured to be 1.86 , of which the free atmosphere component was only 0.37 . The seeing from ∼200 m upward is superior to the leading mid-latitude sites (e.g. Fuchs 1995;Roddier et al. 1990) by almost a factor of two. The results are in good agreement with optical seeing data obtained by a differential image motion monitor on three of the five occasions when the two measurements were performed simultaneously. The boundary layer winds are of katabatic origin, and so we may consider the possibility of exceptional seeing conditions from surface level at other locations on the plateau such as Domes A and C, where there is little or no katabatic wind. In addition, the proximity of the optical turbulence to the focus of a telescope situated at ground level is a highly favourable situation for the use of adaptive optics, since the wavefront spatial coherence scale is related to the altitude of the turbulent layers producing the image distortion. Some comparisons are made between the relevant adaptive optics parameters measured at the South Pole and Cerro Paranal, one of the best mid-latitude sites.