The first indications of returns from above the Fl layer (see top two fonograms of Fig. 3.10) were weakechoes appearing in acattered areas above 850 kmbetween 4 and 5 1/2 mo. These early returns did not have regular shapes which :ouwld yield minimun virtval height and critical frequency data, nor did they show consistency from one record to the next. The F2 echces continued to be irregular and sporadic for about 45 minutes, soon beginning to appear over a great~ er range of heights. There was a prevalence of returns in the region of 600 to 700 ln, as well as in the criginal 850 to 1000 km region, typified by the lower two ionograms of Fige 3.10. (The rigat~hand example also includes one of the leas-frequent returas at tights between these two regions, possibly oblique or the extraordinary ray, but not significant alone). The irregular echoes took shape grad~ ually and became adequate for scaling by 3 1/4 hours after both shots, with two short periods scalable somewhat sooner in the case of Shot 2 (see Fig. 309). The study resulting in the above generalized description cf the F2-layer reappearance led to the hypothesis that there wore maxima of don density at two levels, both extraordinarily high, with the ion density of the higher stratum greater than that of the lower. Under some circumstances, such a condition would yield an F2 trace having the appearance of two distinct layers, This dia actually occur free quently between about 3 1/2 and 4 hours after Shots 1 and 2. (Ses Fig. 3.11, particwarly the lower right Lonogram which shows this stratification within the F2-layer most clearly). An accurate calculation of the electra density distributim implied by the shape of the curve in such ionograms is impossible, but a@ rough approx‘mation of the situation that may have existed will be helpful. In F'g. 3.12 a possible distribution, based on Shot 2 at plus 3 hours 49 minutes, is compared with a purely conjectural nermal distribution which might have been present had the disruption not taleen place. Discussion of the ion movement which may have caused such a distribution appears in Section 4.2. Although F2elayer stratification appeared in many Jonograms be- tween plus 3 1/2 and 4 hours, the two strata hypothesized above were not simultaneously present in earlier records (as the F2 layer was first reappearing). The upper stratum was often missing, probably due to horizontal inhomogeneity and general irregularity of ite io distribution. Such deviaticns from the normal layerelike structure may haves been responsible also for the frequent lack of returns from the lever stratum. It is more likely, however, that absence of the lower stratum can be attributed to transitcry mduction of ie density to values near or below the Fi-layer maximum. Further- more, this latter hypothesis can be extended to explain the compiste absence of F2elayer returns betveen plus 3/4 hour and plus 2 1/2 hours by the assumption that the im distribution had been such that the F2-layer critical frequency was less than that of the Fi-layer. 35 SECRET .