Interpretation of equatorial electrojet irregularities observed with a broad beam HF zenithal radar

Observations of the equatorial ionosphere were performed in Africa with the radar of the Laboratoire de Detection et de Geophysique near the magnetic equator during the International Equatorial Electrojet Year (1993-1994). Th is high-resolution zenithal HF radar uses a broad beam antenna, in which co mplex echo structures are observed. A diversity of irregularities appears i n the 100-500 km radar range up to the F region. Some irregularities appear in range-time-intensity diagrams as a parabolic region between 140 and 180 km with a minimum at noon. This region is only observed for radar frequenc ies between 4 and 5 MHz. At these frequencies, type II irregularities arisi ng from vertical and oblique directions in the 100-140 km range dominate th e spectra, and type I irregularities are difficult to clearly identify. Ray tracing and simulations show that type I irregularities are a possible sou rce of the observed parabolic region. Irregularities from the equatorial "1 50 km region" as observed with the Jicamarca radar appear at slightly highe r radar ranges.