First Whole Atmosphere Nighttime Seeing Measurements at Dome C, Antarctica

Abstract

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.

On the atmosphere for astronomers above Dome C, Antarctica

“…In winter (from March to October) this daily wind speed gradient variation disappears, and the gradient remains permanently and much stronger near to the ground. The resulting shears, combined with the very strong temperature gradient in the same layers, are clearly responsible for the optical turbulence observed in the first 30 m (Agabi et al 2006).…”

“…This is confirmed by the first set of C n 2 balloons equipped with microthermal sensors launched at Dome C during the first over-wintering season (March-August 2005). They show that largest part of the optical turbulence is generally confined in the first 36m (Agabi et al 2006). Probably, such a turbulent inversion layer must be present everywhere on the polar plateau, during all winter, with a thickness that must depend on the katabatic winds and on the penetration in the continent of coastal air streams.…”

“…The DIMM instruments showed a regular deterioration of the ground based seeing quality with the decreasing winter temperature. On the other hand 30 radio-sounding during the dark time have shown a ground inversion layer that is very turbulent, with a thickness of 20-50 m (Agabi et al 2006) During the four previous summers (mid-November to early February), 200 meteorological balloons equipped with standard meteo radiosondes were been launched (Aristidi et al 2005b). They provide the following parameters: altitude, temperature, pressure, humidity, wind speed and direction, up to altitudes generally between 20 and 25 km, and sometimes slightly higher.…”

First single star scidar measurements at Dome C, Antarctica

“…During the 2006 polar winter a DIMM (Agabi et al 2006) was operating on an other platform, 15 m away from the SSS platform, both at 8 m above ice level. The DIMM measures the seeing continuously and it allows a direct comparison with SSS seeing measurements.…”

“…In 2005, the Astro-Concordia station was first opened for winterover during which two DIMMs were set up and 40 meteorological balloons, instrumented for C 2 N (h) and V(h) profiling, were launched. Again, it became obvious that most of the turbulence was generated within the surface layer (Agabi et al 2006), as was found at the South Pole, but with a depth of only 30 m.…”