Nighttime Optical Turbulence Vertical Structure above Dome C in Antarctica

Abstract

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.

Dome C site testing: surface layer, free atmosphere seeing, and isoplanatic angle statistics

“…The best conditions are observed in the summer with a median of 0.54 at an elevation of 8 m, then the seeing degrades to values around 2 in August. This behaviour was noticed since the first winterover (Agabi et al 2008;Trinquet et al 2008a), and seems to occur every year. It was explained by the occurrence, in winter, of a strongly turbulent SL whose thickness is about 30 m (Trinquet et al 2008a).…”

“…This behaviour was noticed since the first winterover (Agabi et al 2008;Trinquet et al 2008a), and seems to occur every year. It was explained by the occurrence, in winter, of a strongly turbulent SL whose thickness is about 30 m (Trinquet et al 2008a). Seeing values at the three available elevations are summarized in Table 1.…”

“…Assuming that sometimes the SL contains a second component above the first main one, this plot shows two probabilities for each value h sl of the height: the highest means all the content of the SL is less than h sl , while the lowest means only the main, most energetic part of this SL is less than h sl . The 3-curve fits on the DIMM data sets provide these numbers at 3, 8 and 20 m. At higher altitudes, a hint can be obtained from the 2005 winter radio soundings (Trinquet et al 2008a). Figure 9 shows the 32 available SL profiles with a vertical resolution of the order of 5 to 10 m (depending on the vertical speed of the balloons).…”

“…Because of the geographical situation, this SL is expected to be much thinner at Concordia (see for instance Swain at Gallee 2006). Indeed, the same radio soundings made at Concordia during the first winter-over season in 2005 revealed a median thickness of 32 m and a seeing likely to be of the order of 0.4 arcsec or better above it, while it is not significantly better than at the South Pole at surface level (Trinquet et al 2008a).…”

“…Both sites of Dome C and South Pole actually exhibit strong winds in winter above 10 km (Marks et al 1999;Trinquet et al 2008a). Figure 19 shows the dependence of θ 0 on the period of the year.…”