The Starfish high-altitude nuclear explosion over Johnston Island (latitude 16° N, longitude 169° W) which took place at 09.00 h U. T. on 9 July 1962, produced very intense and short-lived ionization both locally and at the magnetic conjugate point to the site of the explosion. This ionization density amounted to an order of magnitude increase over the normal level, but on the next pass of the satellite over each of the two regions on the following day the ionization had decayed to practically the level of the day before the explosion. Figure 17 shows the electron density record from the satellite on the first occasion of a pass just west of Johnston Island, which occurred about 61/2 h after the Starfish event. For comparison the corresponding passes are shown on the preceding day and on the following day. The width of this cloud of ionization was a few thousand kilometres along the path of the satellite and the peak density was 3 x 10
5
cm
-3
, which is about 6 times the normal level. The point magnetically conjugate to the site of the explosion is at approximately 15° S, 170° W, and the orbit of Ariel passed close to this point, being slightly to the west, approximately 5 h after the Starfish event. The record of the electron density for this pass is shown in figure 18 with the passes of the two adjacent days for comparison. The enhancements shown in figures 17 and 18 are on approximately the same longitude and at
L
values of 1.18 and 1.10. Further clouds of ionization were encountered a few hours after the explosion near southern apex of the satellite orbit, again on a similar longitude a few degrees to the east and at an
L
value of 6.0. It thus appears that the explosion produced an ionization cloud extending over some thousands of kilometres in altitude, from 25° N to at least 54° S latitude, but very much flattened in longitude. This is not unlike the form which it might be expected the actual fireball of the explosion would take as it expands and then diffuses under the constraint of the geomagnetic field. Further details of the Ariel data relevant to the Starfish event have been reported by Rothwell, Wager & Sayers (1963).
The phenomenology of the mass motion of a high altitude nuclear explosion