AMPLITUDE STATISTICS OF SIGNALS REFLECTED FROM THE IONOSPHERE

Statistical analysis methods used to define the amplitude distribution s of signals returned from the ionosphere are discussed in this paper. Emphasis is placed on determining accurately the parameter B, which i s the ratio of steady to random components present in a signal. Thus B > 1 if the signal is dominated by the steady component, and B < 1 whe n the random components dominate. This study investigates the characte ristics of B for F-region and E-region ionospheric echoes, as well as some types of spread-F, observed at the southern mid-latitude station Beveridge (37.3-degrees-S and 144.6-degrees-E). The results indicate t hat amplitude measurements obtained in approximately 100 s are adequat e for determining B. The results also illustrate some effects that the E-region can have on F-region echoes. It is found that frequency spre ading, the most common type of spreading observed at Beveridge, displa ys strong specular reflections and some signal variation due to interf erence at the leading edge of the F-region echo (i.e. B > 2). Within t he spread echo B fluctuates between 0 and about 1.5 but is typically l ess than 1. The autocorrelation function of signal amplitude has a rel atively large coherence interval, suggesting that this type of spread- F is due to interference of specular reflections from coherent irregul arity structures with horizontal scale sizes of tens of kilometres rat her than scattering from small scale irregularities. A second form of spread-F which would generally be classified as frequency spreading on standard ionograms is actually due to off-vertical reflections from p atches of irregularities which originate south (poleward) of Beveridge . Echoes within this oblique spread-F (OS-F) do not exhibit coherence indicating that the irregularities responsible are of a smaller scale than those producing normal frequency spread. Finally. the phenomenon of spreading occurring on the second hop, but not the first hop trace is studied. It is shown that the form of the second hop echoes can be reproduced using a simple geometric model of ground scatter. The inter pretation is supported by the fact that B for spread second hop echoes is less than 1 whereas it is much greater than 1 for the correspondin g first hop echoes.