SummaryAt frequencies well below the critical frequency, satellite echoes sometimes accompany the night-time F 2 echo, sometimes clearly separated, sometimes overlapping_ In an investigation of these range multiplets, in addition to routine P'f sounding records, continuous virtual range measurements at fixed frequency (at stations of various separations), and measurements of mean intensities, phase-path changes, and directions of arrival, have been carried out_ From a study of the results, certain simple relationships emerge, as first approximations, between the various quantities measured, namely: virtual ranges (group paths), phase-path change, zenith angle, azimuth, intensity, and the time. A parameter, having the dimensions of velocity, appearing in these relationships, has values of which 80 per cent. lie in the range 240 ± 140 km/hr, with directions of which 80 per cent. lie in the range 290 ± 60°. The mean duration of a satellite is 50 min. Satellites occur more frequently in winter than in summer and show a somewhat complex diurnal variation, which is described in detail, and which tends to recur from year to year. It is associated with the general night-time changes in equivalent height of the layer. Strongly reflecting Es shows an inverse diurnal variation. No correlation with geomagnetic disturbance indices could be found.
SummaryA review is given of various hypotheses which might explain the behavioU1' of "range multiplets ", in which satellite echoes accompany the F. echo at frequencies well < below the critical penetration frequency. It is shown that these cannot arise from stratification of the layer. Some few may be due to reflection from the F layer of waves previously scattered upwards by an Es irregularity. The great majority, however, appear to originate in reflections from irregularities in the F layer itself. This is confirmed by the directional observations, multiple. hop echo records, correlations between separated stations, and the usual shape of the range-t~e curves, as described in Part I.Evidence is presented indicating irregularities with fronts up to several hundred kilometres long, with curvatu,res ranging up to 10-0 km-1• The irregularities appear to take various forms, of which the simplest are (a) an inverted trough, around 100-200 km wide, (b) a gap, of roughly the same width, with reflecting surfaces which have, at some points, curvatures up to at least 10 km-l, (0) a step, with height and breadth not exceeding some 10-20 km, again with edges which are strongly curved in parts. Each irregularity probably persists for a period of the order of 1-2 hr and is usually propagated through the F region with a nearly un~~orm velocity of the order of 200 kID/hr. The balance of the evidence indicates that this is not related to any general drift of the region.
SummaryIn the course of investigations of satellite echoes from the F region of the ionosphere. it was noted that the F and Es traces recorded at night, on h't equipment at frequencies well below vertical, are broader than anticipated and tend to change in a characteristic manner as the gain of the receiver is lowered. In this paper, a quantitative explanation of these phenomena is elaborated, based on the postulate of a "rough" ionosphere.This theory leads to a method whereby, from the swept-gain h't records, estimates of roughness index can be formed. These estimates compare satisfactorily, on a statistical basis, with estimates by other methods. The theory is extended to the case of multiple. hop reflections, and to the satellite traces; general agreement with experiment is fonnd. Evidence is presented that the ionosphere appears'rougher when transmitter and receiver are adjacent than when they are widely separated, and a tentative explana. tion is suggested. From the roughness indices, the relative intensities of Z· and O· modeF echoes for Brisbane are computed and the rare appearance of Z-traces on Brisbane records is satisfactorily explained.
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