Near and distant observations of the 1962 Johnston Island high-altitude nuclear tests

Abstract

During the 1962 Johnston Island high‐altitude nuclear tests, USAELDRL operated high‐sensitivity, high‐time‐resolution magnetometers in the Pacific area and in the continental United States, Large loops of up to 100‐km2 area, spin‐resonance (metastable‐helium) magnetometers, and telluric probes were used. Shot Starfish (the July 9, 1962, explosion of about 1½ megatons at 400‐km altitude) produced very strong oscillatory signals of many minutes duration at all our stations (Hawaii, Samoa, Florida, South Carolina, New Jersey, and Maine). The signals consist of several physically different parts—a practically instantaneous broad‐band pulse (observed clearly only in the Pacific area) containing mainly higher frequencies, a strong oscillatory signal starting 1.9 seconds later simultaneously at all stations, a complex part lasting several minutes which is probably a superposition of different hydromagnetic modes, and an extremely long‐period disturbance (observed only in the Pacific area) carrying considerable energy (which we interpret as a hydrodynamic‐gravitational mode). The maximum signal amplitude (occurring within 3 to 5 seconds after the shot), when plotted versus distance from Johnston Island, results in a smoothly decreasing curve. Of the four other high‐altitude nuclear tests above Johnston Island between October 20 and November 4, 1962, which were at various heights but all considerably lower than Starfish, none gave magnetic signals clearly above the noise level at the mainland stations. Three of these events produced clear effects at Hawaii (about 1500 km to the magnetic ENE of Johnston) and at Samoa (about 3400 km magnetically south of Johnston). The signal‐amplitude ratio (Samoa to Hawaii) becomes, for sufficiently low shot heights, a pronounced function of the explosion height, indicating that the magnetic‐signal propagation becomes more and more confined to the north‐south direction as the explosion height is lowered.

The electromagnetic pulse from nuclear detonations

“…There are some basic inferences concerning the source mechanism for these signals that can be drawn from the data, however. A notable feature is the common delay relative to the detonation times of the signals observed at widely different distances from the detonation Bomke et al, 1964]. This delay is consistent with initial propagation of the signal as a modified Alfv•n wave downward through the ionosphere from a generation region near the detonation point, a picture that fits naturally the field displacement model of signal generation.…”

“…The fast onset of this signal component indicates that it is generated beneath the ionosphere, in common with the VLF and higher-frequency radiation from these detonations. Thus it is natural to associate this signal component with the Compton electron model, as has been suggested by Bomke et al [1964]. Finally, Zablocki [1966] has reported measurements in the micropulsation to lower ELF range at a relatively short distance (15.7 km) from two low-altitude detonations of the 1958 Hardtack test series in Nevada.…”

“…Micropulsation records with sufficient time resolution and sensitivity also show a nearly instantaneous response at the detonation time [Abercrombie, 1963;Casaverde et al, 1963;Roquet et al, 1963;Bomke et al, 1964]. If the response of the receiving system extends up to 10 Hz without too much attenuation, this signal component appears as an oscillation in the 7-to 8-Hz range, which corresponds to the lowest Schumann resonance frequency.…”

“…That such observations have not encouraged a greater amount of work on this source Y X• g. g.g. m oZ• mechanism appears due principally to the predominant effect that propagation is thought to have upon the observed signal characteristics [Berthold et al, 1960;Bomke et al, 1960Bomke et al, , 1964Kahalas, 1965;Kovach and Ben-Menahem, 1966;Field and Greifinger, 1967]. There are some basic inferences concerning the source mechanism for these signals that can be drawn from the data, however.…”

Controlled Artificial Generation of Ultra-Low-Frequency Hydromagnetic Waves in the Ionosphere and Magnetosphere

“…The studies were motivated by the possibility of obtaining a relatively inexpensive method for the controlled artificial generation of ULF geomagnetic pulsations.With the exception of the extremely noisy pulsations produced by high-altitude nuclear explosions (e.g.,Green et al, 1962;Crook et al, 1963;Bomke et al, 1964;Bowman and Mainstone, 1964;Kovach and Ben-Menahem, 1966), ULF measurements have been confined to naturally-occurring pulsations. These measurements have given much useful and interesting information about the ionosphere and magnetosphere (e.g.,Jacobs, 1970), and, through magnetotelluric studies, about the II 1 LI structure of the eart, (e.g.,Cantwell and Madden, 1960; Green et al, 1962; Bostick and Smith, 1952).…”