Imaging coherent scatter radar studies of bottomside equatorial spread F

Coherent scatter radar imaging observations have been made of a bottomside spread F layer at Jicamarca. Whereas conventional range-time-intensity (RTI) images cannot resolve the primary waves in the layers, the in-beam images show that the layers are composed of vertical chains of patches of scatterers drifting through the radar scattering volume. These chains outline and delineate primary collisional interchange instability waves with wavelengths of a few km. Different phases of the primary waves ascend at,different rates, and the Doppler spectra of the radar backscatter represent superpositions of spectral lines from different patches on the various phases of the primary waves. G computer simulation of the fluid instability reproduces many features of the radar observations, including the patchy appearance of the backscatter from small-scale, linearly damped waves excited by the nonlinear instability. Some of the patches come into existence by means of a secondary gradient drift instability, leading them to be arranged quasi-periodically in the vertical direction. It is shown that the coherent scatter Doppler frequency spectrum may give a poor representation of the line-of-sight velocity distribution, particularly when a small scattering volume is sampled for a short time. (C) 1998 Elsevier Science Ltd. All rights reserved.