A waveguide interpretation of “Temperate-latitude spreadF” on equatorial ionograms

A waveguide model is presented for propagation of radio waves along elongated irregularities aligned parallel to the earth's magnetic field in the equatorial ionosphere. This theory is applied to analyze the frequency spread ionograms often observed during the equatorial night, particular attention being devoted to the detailed striations of the spread; similar spread-F striations on arctic ionograms are explained by a corresponding waveguide theory. The waveguide theory is consistent with direction-finding evidence that the category of spread F considered in this paper is caused by back-scatter in the north-south plane from irregularities that do not support north-south forward scatter at 50 Mc/s. These echoes are in contrast to those arriving in the eastwest plane from the ‘equatorial spreadF‘ irregularities considered elsewhere, which do support north-south forward scatter at 50 Mc/s. This paper is concerned with propagation in an irregular ionosphere, but not with the formation of the irregularities.