Mid-latitude frequency spread and its association with small scale ionospheric stratifications

Abstract

A variety of ground based radio techniques have provided new information relating to the nature of mid-latitude F-region irregularities responsible for frequency spreading on ionograms. Firstly, an analysis of ionograms covering a restricted frequency band indicates that frequency spreading is primarily caused by duplicate traces which are often unresolved in group path on standard ionograms. Furthermore, where angle of arrival information is available, the duplicate traces are shown to represent reflections from markedly different directions and the spread in critical frequencies is therefore indicative of a horizontal gradient in the peak electron density over a scale size of the order of many tens of kilometres. Secondly, the individual duplicate traces themselves are shown to comprise quasi-horizontal trace (QHT) segments which are unresolved on conventional ionograms and contribute to the diffuse appearance of spread- F traces on those ionograms. Difficulties in attributing these observations to the widely held view that scattering from small scale structures is the causative mechanism are discussed.