A Study of "Spread?F" Ionospheric Echoes at Night at Brisbane. III. Frequency Spreading

Abstract

SummaryVirtual range versus frequency (P'f) records of the ionosphere made at Brisbane (lat. 27·5 oS., long. 152·9 °E.) during 1952 and 1953 have been examined. It is found that occasionally neither the o nor the x mode of propagation penetrates the Fa layer at a unique frequency, the upward sweeping traces either being blurred out over a range of penetration frequencies (diffuseness) or possessing a fine structure (penetrationfrequency multiplicity). Temporal analysis of the occurrence of these. effects reveals that they occur only at night; penetration-frequency multiplicity appearing more frequently in the hours before dawn, while the diurnal distribution of diffuseness has a maximum between 01 00 and 0500 hr in summer and represents a more even distribution between 2100 and 0500 hr in winter. The seasonal distribution has a pronounced peak in the winter months and minima in the equinoctial months. These data are compared with the world-wide picture of these variations as it has emerged from the recent literature.The observations are interpreted in terms of scattering from the clouds of enhanced ionization near the Fa-layer maximum which are believed to be responsible for the scintillation <if radio stars. It is suggested that there isa seasonal vertical movement of these 'clouds, the extent of which increases with latitude.1. INTRODUOTION When the echoes of short pulses of radio-frequency radiation reflected from the F2 region of the ionosphere at night are examined by means of the familiar radar A-scan technique, they are often found to have a configuration not unlike that of a mountain range. The duration of the echo is much longer than that of the transmitted pulse; the successive peaks of echo amplitude mayor may not move relative -to each other, and may fluctuate in amplitude independently as time proceeds or as the frequency of the radiation is varied. This phenomenon reveals itself on virtual range versus frequency (PI!) records as a spreading of the F 2 -region trlltce in range, or penetration frequency, or both, and has consequently become known as " spread-F". In the greater number of occurrences, the spreading is predominantly one of range only (Plate 1, Fig. 1) or of penetration frequency only (Plate 1, Fig. 2), while on a lesser number of occasions both effects are observed simultaneously.Spreading in range usually manifests itself as a multiplicity of discrete F-region traces all of closely similar range-frequency characteristics. The

Spread?F and the Perturbations of the Maximum Electron Density of the F Layer

“…It has been shown (Singleton 1957) that at Brisbane the incidence of frequency spreading is high when the critical frequency of the F 2 layer is low. This result was later confirmed by Bowman (1960b) and extended to include Townsville and Hobart.…”

“…INTRODUCTION Several authors (Gipps, Gipps, and Venton 1948;Little 1951;Kasuya, Katano, and Taguchi 1955;Singleton 1957;Briggs 1958aBriggs , 1958bMartyn 1959) have suggested that the cause of frequency spreading is to be found in irregular spatial variations of the maximum electron density in the F2 layer. The range of penetration frequencies involved fo to fo +l1f is considered to correspond to the range of values of maximum electron density N m to N m +I1N within" view" of the ionosonde.…”

A survey of spread-F

“…Spread-F on the ionogram is a necessary, but not sufficient, condition for the symbol F to appear in the tabulated hourly values. Singleton (1957) made a detailed comparison of ionograms with tabulated foF2 values over a nine month period. He found that over fifty percent of the occurrences of range type spread-F over Brisbane were not indicated by any qualifying symbol associated with the tabulated foF2 values.…”

“…The degree of the spread condition may vary from a slight spread not obscuring the critical frequencies, as in Figure 15, to a heavy blanket of spread, as in Figures 3 an<i 1 to k Singleton (1957) 1 to 8…”

Broadband radio-star scintillations. Part 1. Observations

“…It has long been realized that r adio-star scintillations and the ionospheric phenomenon oJ spread-F [Singleton, 1957] are r elated and probably have a common origin [Ryle and Hewish, 1950;Little and Maxwell, 1951]. Consequently, associations were sought b etween broadband scintillations and various properties of the ionosphere.…”