Ionospheric Features Associated with the Fine Structure Recorded on Some Mid-Latitude Spread-F Ionograms.

Abstract

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 fixedfrequency ionograms suggest that dynamic changes are occurring to ionospheric structures over and above those normally expected during the passage of a travelling ionospheric disturbance.

Short-Term Delays in the Occurrence of Mid-Latitude Ionospheric Disturbances Following Other Geophysical and Solar Events.

“…1(d), recorded 15 minutes earlier does not reveal the presence of duplicate traces. It seems likely that AGW non-linear effects (Bowman, 1991c) are responsible for the generation of field-aligned irregularities which if sufficiently developed can be detected on ionograms (Bowman and Hajkowicz,199 1). Figure 1(f) shows such an ionogram where the field-aligned irregularities produce traces at frequencies in excess of f0F2 with a characteristic straight underedge.…”

Nighttime Mid-Latitude Travelling Ionospheric Disturbances Associated with Mild Spread-F Conditions.

“…However the real occurrence rate at night (as detected by ionograms) will be masked because it becomes very difficult to identify TIDs at times of diffuse spread-F occurrence. Details of trace complications on ionograms at times of intense spread-F activity are given in a companion paper (BOWMAN, 1991a). In any case until more recent times diffuse spread-F ionograms were thought by many investigators to result primarily from scattering from small-scale structures with TIDs not considered as being involved.…”