This is known as a "G condition" .2/ it should be noted that the G conditia adduced here does not indicate any major difference between the effects of IVY Mike and CASTIE l and 2. It may be seen from Fig. 3.9 that, during Operation - IVY, normal values of F2 critical frequency were much greater than . during the first month of Operation CASTLE. The Fl critical free quencies, however, were of the same order for these two periods. Consequently, while observation of the F2 layer may have been impossible following CASTIE 1 and 2 because of a G conditim, a comparable depression of the F2elayer critical fraquency resultig from IVY Mike would not have been sufficient to cause such condition. Figs. 3.13 and 3.14 give, for Shots 1 and 2 respectively, pequences of ionograms showing te descending virtual beight of the F2 layer as it returned to normal. There are indications of obliqus returns, particularly in the last picture of Fig, 3.14, evidencing effective layer tilt. 30302 Shot 3 No effect on the height or critical frequency of the F2 layer was observed after Shot 3. This is consistent with the inference drawn from Operation IVY that the minimm energy release required to produce the rising-F2 phenomenon is apparently greater than 1/2 megaton (see Section 5.2 of Reference 1). 30303 Shot 4 The effect om the F2 layer above Rongerik following Shot 4 was less pronounced than that observed following Shots 1 and 2. The F2 layer was observable almost continuously after the absorption that has been attributed to tha effect of the shock vave om the D region had abated. Scaled values of virtual height and critical free quency are shown in Fig. 3.15. Also plotted for comparison are maximun, minimum, and median values of data taken over a period of eleven s pricr to the shot. It may be seen that, following tho shot, virtual heights were consistently greater than the height a/ 4 "G Condition® is an imospheric abnormality, observed occasiconally in all parts of the world, usually associated with a magnetic etorm. The condition is simply that the maximum density of the F2 ‘layer (as measured by its vertical~incidencse critical frequency) is equal to or less than that of the wmderlying Fl. When an analyst finds an jonogram 4a which the Fl layer appears and the F2 layer does not, hs suspects a G condition. However, positive identification is made only after studying records taken immediately preceding or following the suspected condition. If these records show an F2 critical frequency just greater than the Fl critical frequency, a trend is established and it is a safe assumptio that a G condition existed. 39 SECRET