NNT ar ae ae mmee 303 Nee aeee ee ee EFFECTS ON THE F2 LAYSR - As discussed in Section 3.1, absorption at lower levels pre~ cluded observation of the F2 layer, both at Site Elmer and at Rongerik Atoll, for some time after each shot. The greatly prolongedabsorption at Elmer rendered that station useless in studying the F2-layer effects. However, sufficient data were obtaired at Rongerik to substantiate the rising F2elayer phenomenon first observedafter Shot Mike of Operation IVY. Since the ionosphere above Rongerik exhibited differences following the various shots, the results of each detonation are examined here separately, except that Shots 1 and 2, which produced similar effects, are discussed together. , 323.1 Shots 1 and 2 The similarity between the F2-layer effects of Shot 1, Shot 2, and Shot Mike is evident in both the virtual height and critical free quency curves of Fig. 3.9, wherein these two characteristics are plotted against elapsed time measured fron zero hour. (For comparative purposes such a presentation is more useful than curves plotted against time of day; it is also justifiable because each of the three shots involved was detonated shortly after ionospheric sunrise and so condi~ tions in the F2 region were similar.) In each case, the virtual height _ of the layer rose to well above normal and returned to normal at a similar rate and after a nearly equal tine interval, Likewise, the critical frequency was depressed below normal by about the sam ameunt after each shot. Thus, the F2 region above the Rongerik station, 243 km east of CASTLE Shots 1 and 2, seems to have wdergone perturbations physically similar to those which had been observed at Bikini, 360 km east of Shot Mike, Operation IVY. 4s indicated in Fig. 3.9, no returns from the F2 layer appeared in the Rongerik ionograms until about 2 1/2 hours after Shots 1 and 2. Complete absence of returns near shot time has been attributed to equipment inadequacies and to increased atsorption associated with arrival of the shock wave in the lower ionosphere (see Section 3.1). Starting at about plus 3/4 hour, the absorption effect abated and sporadic-E and Fl traces took form (se. Section 3.2). Neither absorp- tio nor blenketing by sporadic-5, then, is the cause of the failure of any echo to return from the F2 layer at Ragerik between plus 3/4 hour and plus 2 1/2 hours. In order to investigate possible explana- tions for this iack of F2 returns, let us now consider the manner in which the F2 layer began to appear in the lonograms. 33 SECRET