The influence of wind on anisotropy in optical turbulence

Measurements with Hartmann Turbulence Sensor revealed anisotropy in optical turbulence. The orientation of this anisotropy was found to be frequently correlated with the direction of the wind. A Hartmann sensor was employed in two data collection campaigns in the summers of 2016 and 2017. In 2016 data was collected on a 200 meter path over a grassy field; in 2017 a 1 km path over an asphalt runway was employed. Similar results were obtained in both cases. The sensor is 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. A 2 mW HeNe operating at 632.8 nm was placed at the other end of the experimental path and was beam expanded to overfill the telescope aperture. The sensor recovered tilts across each subaperture from centroid shifts of the imaged HeNe spots. After global tilts were removed, anisotropy was gauged from the eccentricity and orientation of a bivariate Gaussian fit to this two-dimensional data. The anisotropy observed was modest in strength (<30%) and only rarely was this anisotropy found to be aligned with the horizontal and vertical directions. This result breaks the apparent symmetry of the experiment, but a cross-wind also breaks this symmetry, so this effect was examined, and it was discovered that the anisotropy direction was well correlated with the wind direction. Physically these results can be explained by turbulent eddies rising from the ground and being distorted by the crossing wind.