Refractive index gradient of the atmosphere at near ground levels

Abstract

We experimentally investigated the mean refractive index gradient and its fluctuations due to atmospheric turbulence. The horizontal and vertical gradient components were directly measured by the corresponding displacements of thin laser beams propagating horizontally near the ground over short paths. A strong anisotropy was found when average values were observed as a function of time. This anisotropy can be related to the local convective air movement from the ground, irregularly perturbed by the wind. As a test, wind velocity was simultaneously measured and correlated to refractive index gradient measurements. The fluctuation PDFs (probability density functions) of the gradient horizontal component fluctuations show marked exponential tails, independently of the time-scales τ over which the averages and the corresponding fluctuations are evaluated. A similar behaviour is found for the vertical component only when very small values of τ are considered, showing that isotropy conditions hold for very short time-scales.

Acknowledgments

This research was supported by a PAIS 1999–2001 (Progetti Avanzati ed Iniziative di Sezione) of the Istituto Nazionale per la Fisica della Materia (INFM). We gratefully thank Dr Rodolfo Guzzi and Dr Lorenzo Grassi (CNR—Consiglio Nazionale delle Ricerche, Area di Bologna, Italy) for having put their working facilities at our disposal and for technical assistance. The precious collaboration of Professors Sun Yi Yi and Li Zhi Ping (Yantaii University, PR China) and Gianni Paoli in making the measurements is also greatly acknowledged.