The absolute scattering cross section at 50 MHz of equatorial electrojet irregularities

We have made carefully calibrated radar scattering measurements by using the large 50‐MHz Jicamarca antenna. Typical results from the altitudes of maximum echo power for the vertically directed beam are σ radar ∼1–2 × 10 −10 m −1 for strong daytime electrojet conditions with type 1 irregularities present, with values a factor of 10 or so smaller during moderate conditions when only type 2 are observed. These cross sections, which are very large in comparison with those for incoherent scatter (σ radar ≃5 × 10 −18 m −1 for an electron density of 10 11 m −3 ), are not nearly large enough, however, to cause pseudo‐absorption events on riometer records, as was suggested by D'Angelo in 1976. If a model for the k dependence of the irregularities is assumed, it is possible to relate the radar data, which correspond to a single point in k ‐space, to in situ measurements of δn/n o , which correspond to a single point in physical space (an integral over the k ‐spectrum). The comparisons are reasonable, but depend significantly upon the assumed large scale cutoff for type 2 irregularities and the small scale cutoff for type 1.