Measurements of anisotropy in optical turbulence

A Hartmann Turbulence Sensor was employed along a 200 meter path over a grassy field to investigate turbulence anisotropy. This sensor was based on a 16" telescope with a 32 × 32 subaperture array featuring 700 active subapertures and a camera operating as fast as 8000 frames/sec. Data was recorded at different times of the day over the course of a 5-week period in 3-12 hour capture periods. Anisotropy was gauged by both differences between horizontal and vertical tilt variances observed on subapertures and also on variances of tilt differences between subaperture pairs. This latter differential tilt approach is expected to be largely immune to base disturbances. Unsurprisingly, during the day the turbulence appeared to be largely isotropic. During the evening neutral events, (at times near sunset) the turbulence showed a statistically significant anisotropy with tilt variances and differential tilt variances in the vertical direction exceeding those on the horizontal by roughly 5%. The anisotropy observed is consistent with the atmosphere being arranged into a vertically layered structure. At times near sunrise, where limited data was collected, this effect appeared to be weaker. No clear conclusions are drawn for the nighttime case, where while the turbulence was generally lower than that seen during the day, the measures of anisotropy appeared to be quite noisy. The results of this experiment are in general agreement with those of Sanchez et al. (referenced herein), who used a largely identical instrument at Kirtland AFB, NM, but with an environment and collection geometry which were quite different. Most models of surface layer turbulence presume isotropic conditions, this research shows that this presumption isn't always correct and a small correction is all that would be needed to create more realistic models.