Very low frequency disturbances and the high-altitude nuclear explosion of July 9, 1962

Abstract

On July 9, 1962, a 1.4-megaton nuclear device was exploded at an altitude of 400 km above Johnston Island in the central Pacific. The burst perturbed the relative phase of three frequency-stabilized VLF transmissions on propagation paths shielded by the earth from the direct effects of the explosion. The temporal variations of these three disturbances differ in major respects. On the NPG, Jim Creek, Washington, to APL/ JHU path, the onset and maximum were essentially instantaneous, a characteristic which generally fits the burst-related neutron-decay model of Crain and Tamarkin in which sudden ionization in the VLF altitude region is produced by neutron-decay electrons geomagnetically guided into the lower ionosphere. In addition, the NPG-APL variation shows a secondary perturbation having a 10-second period and stemming from a hydromagnetic disturbance associated with temporarily trapped neutron-decay protons of 0.4 Mev. In contrast to the NPG-APL perturbation, and among other differences, the perturbations to the transmissions from NBA (at Balboa, Panama) and WWVL (at Boulder, Colorado) are marked by a delay in the onset and maximum. The disturbance observed for the NBA-APL path, which lies almost along a geomagnetic meridian, results from ionization of the lower ionosphere by electrons that were produced in the radioactive decay of fission fragments and that formed an artificial radiation belt. Here there is good qualitative agreement between the temporal variation of the VLF perturbation and the energy contained in the stream of trapped fission-decay electrons drifting azimuthally from the burst region over Johnston Island to the NBA area. The temporal VLF phase variation for the WWVL-APL path (which lies along a line of nearly constant geomagnetic latitude) indicates that the major part of this perturbation is due to ionization resulting from the effects of geomagnetically trapped neutron-decay electrons. A relatively early and small part of this disturbance also results from the contribution of trapped fission-decay electrons.