Atmospheric turbulence at the South Pole and its implications for astronomy

Abstract

To investigate the low-atmosphere turbulence at the South Pole, we have measured, using a SODAR, the temperature fluctuation constant ( CT2) during winter, as a function of altitude up to 890 m. We found that the turbulence was on average concentrated inside a boundary layer sitting below 270 m. While at the peak of winter the turbulence was stable and clearly bounded, during other seasons there was a more complex turbulence profile which extended to higher altitudes. We found that this behaviour could be explained by the horizontal wind speed conditions whose altitude profile closely matched the turbulence profile. We also observed the presence of a vertical wind velocity change of direction at an altitude range corresponding to the turbulent region. The turbulence gives rise to an average seeing of $1.73^{\prime\prime}$, which compares poorly with the best astronomy sites. The location of the turbulence, however, means that the seeing quickly decreases above the boundary layer (dropping to $0.37^{\prime\prime}$ above 300 m). We also have recorded the largest isoplanatic angle ( $\theta_{\rm AO}=3.3^{\prime\prime}$) and the longest coherence time ( $\tau_{\rm AO}=2.9$ ms) of any ground-based site.