(The F2 echo alluded to may be found between 400 and 500 km and be- tween 3 and 4 mc for the two earlier pictures and just above 4 m for the later one. In the earlier pictures, the two "smudges" are the magneto-ionically split ordinary and extraordinary F2 traces, in the later one, only the ordinary return is clearly delineated.) The occasional appearance of the F2 layer in a near G condition renders it a safe assumption that a G condition existed when the F2 layer did not appear. of the F2 layer above Rongerik following Shot 5 was again abnormally low, as it was after Shots l, 2, and 4. The virtual neight at which the F2 layer occasioally appeared was of the order of 350 to 600 kn. This range is definitely higher than normal, but must be accepted with some reservations, as, under these conditions, the trys height may be appreciably less than the virtual height (scaled from the ionograms) due to the severe retardation that a siztal with a frequency near the © Fl critical frequency suffers in .ecssing through the Fl layer. Our knowledge, ther, of the distribution of ionization in the F2 region following Shot 5 is guite limited. A further limitation is due to the curtailment. perforce, of data after plus 2 hr 37 min, 32305 : Thus, the data show that the maximum io density . Shot 6 Unfortunately, there are no recorded observations of the ionosphere following Shot 6. The station at Rongerik was dismantled prior to the shot for administrative reasons. At Site Elmer, although visual observations of the oscilloscope were made, the photographic apparatus failed to record. From the notes made during visual observations at Elmer, it seems that absorption effects prevented F2-layer returns from appeare ing until plus 2 hr 5 min. Sporadic E appeared then and may nave blanketed returns from higher levels until plus 2 br 39 min, at which time faint F2 returns appeared above 350 km between 5 and 6 m. A clearer, more layerelike echo soon appeared, and its virtual height dropped from 350 km to 300 km between plus 2 hr 47 min and 3 br 7 min, renaining near there wtil plus 3 hr 33 min, and dropping to 2&5 km at plus 4 br. During that period, the critical frequency increased from about 5.7 m at first to about 6.7 m at 3 br 7 min, 7.2 m at 3 hr 33 min, and 7.5 to & m at plus 4 hr. The scanty visual cata indicate the possibility that Shot 6 had an effect on the F2 layer above Elmer. The virtual height, at +2 hr 47 min, was slightly higher than normal, and the 50 km decrease in the ensuing 20 min was contrary to the normal tendency to increase (by about 50 lan per hr) at that time of day; the value of 28 km (at+4 br) was about 90 km below normal height. In addition, the 1 me increase ef critical frequency in 20 min was much more rapid than the normal rate of 0.4 mo per hr between 0900 and 1000 hr; also, critical free quencies above 6.7 mo, as indicated between + 3 and 4 hr, were greater than any occurring at that time of day during the preceding tan days. 50 SECRET :