
%0 Journal Article
%A Singleton, DG 
%T A Study of "Spread?F" Ionospheric Echoes at Night at Brisbane. III. Frequency Spreading
%J Australian Journal of Physics
%D 1957/03/01
%V 10
%N 1
%P 60-76
%K 
%R https://doi.org/10.1071/PH570060
%U https://www.publish.csiro.au/paper/PH570060 
%X Virtual range versus frequency (P'f) records of the ionosphere made at Brisbane (lat. 27·5 °S., 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 F2 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 (penetration-frequency 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 0100 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. 
