抄録
The main aim of this paper is to discuss and illustrate why the occurrence of diffuse mid-latitude spread-F ionograms suggests scattering from small-scale ionospheric structures as the primary cause of the spread, contrary to present ideas involving duplicate traces as the basic trace structures on sonograms. A number of factors which modify duplicate traces and tend to produce diffuseness are described. Sometimes these factors, which mainly involve interference effects due to overlapping traces, are so dominant that the diffuseness obscures any trace of the duplicate traces. In addition an important factor is the periodic replacement (sometimes recorded) of the quasi-horizontal-trace (QHT) segments of the duplicate traces (reported earlier for daytime events). These effects which are detected on fixed-frequency ionograms suggest that dynamic changes are occurring to ionospheric structures over and above those normally expected during the passage of a travelling ionospheric disturbance.
