Characteristics of spreadFat high geomagnetic latitude

Abstract

A detailed analysis of the fading observed during spread F at Ithaca, New York (74° dip), shows that steady components are present at all ranges within the echo proper. Accordingly, it seems unlikely that theories relying on random scattering processes can properly explain the pronounced spread in range observed. To investigate the mechanism responsible for spread F, a lobe-swept interferometer was designed to estimate the width of the angular spectrum of radio waves reflected from the ionosphere under spread-F conditions. This instrument was operated at spacings from 5½ to 12½ wavelengths. Data obtained with it indicate that the type of spread F-observed at high geomagnetic latitudes has a narrow angular spectrum. These observations are in substantial agreement with recent theories that attribute long delay times to the guiding action of field-aligned irregularities within the F layer. The correlation of spread F (as observed on an ionospheric sounder at Ithaca) with strong scintillation of the radio source Cygnus A at upper culmination is found to exceed 0.8. The height of the irregularities responsible for radio-star scintillation is not known accurately; there are indications, however, that the irregularities lie above the F-layer maximum, and it is tempting to imagine them extending right through the F region on nights of spread-F activity.