for the field report on Shot 4 alone, but used here since these curves typify all shots of very large magnitude. In these figures, the minimum frequency reflected from the ionosphere and detected at each station is plotted against lcecal time, using a light line. The standard symbol "B" is used to denote times when complete absorption appareatly prevented any lonospheric return; occasionally, for very brief intervals during these periods, an extremely weak signal was perceived, hence the symbols are located in the vicinity of the frequency involved. While it is true that there are other factors influencing the _ an minimun frequency value, it is principally a fimction of the degree of absorption. This may be inferred from the normaleday average~value curves of Figs. 3.1 and 3.2. The trend of these curves indicates that the minimum detected frequency is greatest when the stm is highest and the absorption @hich is dus to the icnizing radiation from the sm) is also a maximum. . —-—---~ Minimum detectable frequency, day of Shot 4 Ditto - occasional values during periods of otherwise complete absorption Ditto - rate of change unknowm X——%——-x Average values 8668 - ; Tee For 12-day period (6 days pre- Max and min observed values ~6 E p 88s ceding and following Shot 4) 8 9 —_ ~ o ce es 2 oe & E = = = e 3 a H $ 06 ! 1 08 i l 10 I Local 1 £ 2 time (heurs} ! 14 1 1 16 18 Fige 3.2 = Minimum Frequency Detected at Site Elmer for Day of Shot 4, Compared with Normal Valves 2h SEC eT