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.
The Es region as recorded on routine h'f records taken at Brisbane (latitude 27°.5 south, longitude 153°.0 east) between June 1943 and December 1949 has been studied. At all seasons the critical frequency is lowest at dawn. In summer months, the critical frequency reaches a maximum at about 10h and then declines gradually, remaining high until after midnight. In winter, the rise is slower and the maximum critical frequency occurs around 14h, dropping markedly by sunset. In general, a high Es critical frequency is accompanied by blanketing of F echoes up to a comparatively high frequency; but, whereas the occurrence of high critical frequencies shows a summer maximum and a winter sub‐maximum, the occurrence of high blanketing frequencies is least frequent in winter. The observations suggest that there are two distinct types of Es common at Brisbane—one formed at greater heights and descending to its final position, the other formed in situ. The first, the predominant type in summer, blankets strongly and has probably a uniform ionization density; but the second, the winter type, blankets little and probably has pronounced lateral irregularities. No evidence of correlation could be found with sunspot numbers, ionospheric storms, or meteor occurrence frequency, and the conclusion is reached that the Brisbane Es is not predominantly of meteor origin. There is some slight evidence of correlation between the constant‐height type of Es and F region diffuseness.
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.
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