The nature of ionospheric spread- F irregularities in mid-latitude regions

Abstract

Initially, this paper considers earlier experimental results (some of them hitherto unpublished) obtained by making observations on signals returning from mid-latitude spread- F irregularities. These results suggest associations between spread- F irregularities and nighttime travelling ionospheric disturbances. Statistical analyses are then described which investigate the spread- F phenomenon at a number of mid-latitude stations with approximately the same latitudes but distributed over a range of longitudes. An east-west movement of spread-F irregularities is revealed when the occurrence at these stations is considered relative to days of enhanced occurrence at a particular station. All the experimental evidence presented in the paper supports the idea that the appearance of mid-latitude spread- F ionograms results primarily from specular reflections from relatively-large-scale structures which can be imagined as being in fact nighttime travelling ionospheric disturbances. These are, in turn, possibly related to internal gravity waves in the neutral atmosphere. It is suggested that the small-scale ionospheric structures (which are undoubtedly also present) are effective in inhibiting some of the specular reflections thus contributing to the diffuse nature of some records. This idea is quite contrary to the generally-accepted view that the spread- F traces are a direct consequence of scattering from these small-scale structures.