Small-scale stratifications associated with daytime travelling ionospheric disturbances in mid-latitude regions

Limited-range ionograms have revealed the presence of quasi-horizontal trace (QHT) segments during daylight hours. This subdivision into component segments of both main traces and the extra traces produced by travelling ionospheric disturbances (TIDs) is seen regularly on limited-range ionograms in mid-latitude regions. F1-region TID wavetrains of 5 or 6 cycles are reported with periods around 10 min. The identification of wavetrains when extra traces are absent is made possible by QHT segments which are found as a function of time to move to lower frequencies in the manner similar to that found for the extra traces. Furthermore particular QHT patterns are ephemeral. It is suggested that the segments result from small-scale stratifications produced by saturation effects of atmospheric gravity waves. Daytime ionograms also record spreading on first-and second-hop traces as a result of very weak signals probably produced by reflections from field-aligned columns which could be embedded in the stratified regions. This spreading appears on ionograms as qusi-horizontal (QH) patches