The absolute scattering cross section at 50 MHz ofequatorial electrojet irregularities

Abstract

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 σradar ∼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 (σradar ≃5 × 10−18 m−1 for an electron density of 1011 m−3), are not nearly large enough, however, to cause pseudo-absorption events on riometer records, as was suggested by D\u27Angelo 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/no, 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