frequency, was found to fall below normal values (see Reference 1). Although, during previous operations, ionosphere recorders had been operated following at least a dozen atomic explosions, occurring at ‘various hours, seasons, and lccations and under various ionospheric conditions, none of the records obtained exhibited this phenomenon of the rising F2 layer. Since the yield of Shot Mike was muck greater than that of any previous test explosion, it was concluded that the effect was a consequence of the magnitude of the shot. It was recom mended that, inasmuch as Operation CASTLE would include shots of comparable magnitude, confirmation should be sought and that, by locating two ionosphere recorders in different directions from the source, a better insight into the mechanism might be obtained. . & possible explanation of the phenomenon was given, based on the assumption that the blast-wave energy, arriving at ionospheric heights as a shock wave, was absorbed there and converted into thermal energy, thus raising the average temperature of a large regio of the upper atmosphere, causing expansion and rising of the heated volume. If the charged particles were to move in the same direction as the neutral air molecules, the F2 layer would rise with the rising air mass, and the depressed critical frequency would be explained by the decreased electron density resulting from expansion. However, the effect of the earth's magnetic field on charges moving across it casts doubt on this simple explanation; so a more involved hypothesis was necessary to account, in part, for the phenomenon. Recent publication of new ionization drift theory has made possible a more-complete ex- planation (see Section 4.2). In order to test the theory given for IVY Mike results, it was at first proposed that two ionosphere recorders be so located that Propagation of the blast wave to the two operating sites would take place across and along the earth's magnetic field, respectively. Howe ever, it was not logistically practicable to obtain such locations. Rongerik Atoll (about 150 miles east of the Bikini shots) and Eniwetok Atcll (about 200 miles west of Bikini) were selected as the best avail-~ able locations from which to obtain lonospheric data that might result -in a better understanding of the phenomenon, 1.2e2 Effects at a Great Distance Irregularities found in ionograms recorded at Guam, Maui, and Okinawa were interpreted as due to an indirect effect of Mike shot on the F2 layer. From the times at which the irregularities were first observed, it was hypothesized that an ionospheric disturbance, ini-~tiated by the blast, was propagated horizontally in the F region at a velocity of approximately 13 km/min (about two thirds of the speed of sound near the earth's surface). This velocity was shown to be with- in the ranges reported by several observers for naturally caused disturbancese However, this appears to have been the first time that a ‘traveling ionospheric disturbance emanating from a known source ras reportede 18 SECRET