Scattering of ordinary waves close to the cut off layer and microturbulence measurement with a reflectometer

Investigations relating to scattering processes near the cut off layer, where WKB approximation is not valid, are reviewed. The potentiality of reflectometry experiments to measure the spatial localization and statistical characteristic of small scale low frequency density fluctuations is discussed. In order to calculate the scattered field, a slab model and a wave propagating along the density gradient are considered. The WKB analysis is reviewed and a perturbative expansion is presented