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.

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