HF RADAR OBSERVATIONS OF EQUATORIAL SPREAD-F OVER WEST-AFRICA

New experimental data depicting equatorial spread-F were taken during an HF radar sounding campaign in Korhogo (Ivory Coast, 9 degrees 24N, 5 degrees 37W, dip 4 degrees S). Range-time-intensity maps of the rada r echoes have been analyzed to identify the signatures of density depl etions and bottomside spread-F. Density depletions are well known feat ures of equatorial spread-F, and are believed to emerge after the deve lopment of a Rayleigh-Taylor instability on the bottomside F-layer. A simple model is developed and used to simulate the flow of density dep letions over the radar field of view. The simulation permits an interp retation of the data that yields the zonal flow velocity as a function of local time. Comparisons with previous measurements are undertaken to assess the consistency of the computational results, and qualitativ e arguments are presented to identify bottomside spread-F. Using the c omputational results as reference, a morphological study of ionograms showing spread-F is undertaken which reveals the specific signature of bottomside spread-F on ionograms recorded just after sunset.