Anisotropic field‐aligned ionization irregularities in the ionosphere near the magnetic equator

During the 1958–1959 International Geophysical Year, a three-frequency, swept-azimuth backscatter sounder was operated by Stanford University at Huancayo, Peru, in cooperation with the Instituto Geofisico de Huancayo. An unusual echo, which has been shown to be due to the presence of field-aligned ionization irregularities in the ionosphere, was discovered early in the program. These equatorial scatter echoes (EqS) are observed on over 90 per cent of the days (at 18 and 30 Mc/s) and exhibit a diurnal variation very similar to that of the equatorial electrojet. By means of the backscatter pulse observations, in conjunction with the Booker theory of backscattering from anisotropic field-aligned irregularities, it has been possible to determine the size and extent of the scattering region. The E-region echoes originate at heights between 100 and 140 km. At times, however, the upper limit may extend to 200 km or more. F-region echoes have occasionally been observed which appear to originate at heights corresponding to the maximum ionization density of the layer. Observations of the strength of the EqS echoes indicate that the mean square fractional deviation of the electron density is between 10−3 and 10−5. The discovery of the presence of equatorial field-aligned ionization irregularities has made it possible for the first time to describe adequately a number of equatorial ionospheric phenomena, including the mechanism of ‘equatorial-type’ sporadic E.