Arecibo ISR observations and VHF coherent backscatter data collected with a radar situated in southern Puerto Rico are examined to understand the causes of quasi-periodic variations in VHF returns from lower E-region heights (∼93 km). It is found that Bragg scale density irregularities causing the observed VHF backscatter are formed and propagated horizontally in a density perturbation field localized at the upper boundary of a tidal ion layer. The density perturbation has a 7 km horizontal wavelength, propagates in southwest direction, and it is not aligned with the geomagnetic field. Thus the production mechanism of the observed density perturbation cannot be the gradient-drift instability often invoked to account for the generation of mid-latitude sporadic E radar echoes. Nevertheless, secondary gradient-drift instability operating on the gradients of the southwest propagating perturbation remains the most likely cause of Bragg scale electron density waves responsible for the VHF returns.
All Science Journal Classification (ASJC) codes
- Earth and Planetary Sciences(all)