A review of some recent work on mid-latitude spread-F occurrence as detected by ionosondes.

The characteristics of mid-latitude spread-F occurrence are reviewed (in the light of some recent publications) in terms of four different scale sizes. The term "macroscale" has been used to describe significant height rises and electron-density depletions which in general precede spread-F occurrence. TID wavetrains in the form of wavelike modulations of these height rises are termed "large-scale" structures. The experimental evidence suggests that off-vertical reflections from tilts associated with these wavetrains are responsible for the basic components of spread traces on ionograms. Also, it is proposed that "medium-scale" structures at the F2-layer level are a result of stratification effects, and finally that field-aligned irregularities which sometimes produce weak spread traces, be termed "small-scale" structures. Attention is drawn to the fact that structures similar to those proposed here for mid-latitude spread-F appear to be related to other phenomena in equatorial and sub-auroral zone regions. It is suggested that the breaking of atmospheric gravity waves has a central role to play in the development of these mid-latitude spread-F structures.