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.

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The horizontal component of the geomagnetic field, measured on the ground at two stations near the path of totality, reached the greatest departure from its normal value shortly after maximum eclipse in the nearby E layer. The magnitude of this departure is consistent with the generally accepted hypothesis that the eclipse‐induced current system is located near the 100‐km level. A determination was made of the lag from the time when the E‐layer eclipse occurred at the theoretically optimum location until the time of maximum observed magnetic effect, with corrections for induction delays. Using this time lag, along with the observed decrease in the E‐layer ionization, the effective ion‐recombination coefficient and the percentage of ionizing radiation remaining at totality were determined. The recombination coefficient was found to be 5.5 (±1) × 10−8 cm³ sec−1 and the residual coronal radiation 15 (±3)%.

A new approach was used to study the worldwide effects recorded by standard magnetometers a few minutes after the Starfish detonation. The magnitudes of the total disturbance were plotted on a world map and, by the use of standard contouring techniques, global lines of equal signal strength were obtained. In this system of isopleths, an outstanding feature is a pair of clearly defined maximums located near the geomagnetic meridian of the source and centered at geomagnetic latitudes of about 45° North and South, whereas there is a relatively low value near the shot location. Another innovation was the introduction of a coordinate system based upon the geomagnetic field direction at each station; one axis (called a) is along the dipole field line and the other two orthogonal axes lie in the plane (called p) perpendicular to the dipole field. The close resemblance between the isopleths of the p component and those of the total disturbance (at 3 min postshot) indicates that the major portion of the signal was in the p plane at that time. The a pattern, while more complicated than the p, clearly exhibits certain symmetries that are described and tentatively identified. To account for the two p maximums a possible mechanism was advanced, namely, that the pattern is the result of coupling of energy from the isotropic, modified Alfvén mode into transverse, pure Alfvén waves in regions where favorable coupling conditions exist.

During August and September 1960, a very large wire loop (area about 100 km 2) was used to record different types of earth-magnetic micropulsations at Baxter State Park, Maine. The average values of the magnetic amplitudes were compared with the average values of the electric amplitudes of the same types of micropulsations. There is strong indication that for micropulsations in the 1-cps range the ratio E/H is about 100,000 ohms, whereas for micropulsations in the range between 0.1 and 0.01 cps, E/H is of the order of 10 ohms. The distinction is made between electric-dipole radiation, produced by free electric space-charge oscillations, and magnetic-dipole radiation, generated by closed electric currents. The former are assumed to be connected with the absorption of auroral particles in the ionosphere, whereas the latter are the result of the absorption and reflection of hydromagnetic waves in the lower ionosphere.Simultaneous recordings of the same micropulsation events with two different loops showed an amplitude ratio that corresponded to the loop sensitivity ratio only for the longer micropulsation periods (10 to 100 sec). The micropulsations around I cps often showed a much stronger signal on the small than on the large loop. This seems to indicate that the sources for the 0.1-to 0.01-cps micropulsations are large (at least of 10-km diameter), while the 1-cps sources are very often small (of 400-to 500-m diameter).