Studies on sudden commencements of geomagnetic storms using IGY data from United States stations

This report contains studies of geomagnetic storm variations recorded at a network of seven IGY stations in the United States together with records for other observatories operated by the United States. The distance between adjacent stations of the network ranges from 360 to 510 km. Geomagnetic variations at these stations were usually quite similar, as would be expected. During magnetic storms, however, remarkable differences occurred even between adjacent stations. About half of the sudden commencements of the horizontal component showed different shapes at adjacent stations for the same magnetic storm. From the data obtained from these results, and from observations at more distant stations, the behavior of sudden commencements is examined. The data also show that the variations of the vertical component of the sudden commencement differ in ways suggesting notable irregularities of the earth's conductivity in the central part of the United States. In addition, occasional examples of bay-shaped variations during magnetic storms, which are quite nonuniform over the closely spaced net, are illustrated and analyzed.