Geomagnetic pulsations are small oscillatory changes in the earth's magnetic field. Their periods fall in the ULF (ultra low frequency) band and range from a second up to 600 seconds. First reported in the last century, significant progress in understanding the origin of these surprisingly sinusoidal signals only followed J. W. Dungey's suggestion, in the early 1950's, that they might be due to hydromagnetic waves in the earth's magnetosphere. The 1960's saw an accumulation of evidence, but in the 1970's the field blossomed.
A variety of factors were responsible. Most important was that theories of large-scale structure of signals were developed just as the first arrays of magnetometers were measuring the north-south signal structure along lines of longitude. Theory and experiment agreed probably better than theorists or experimenters had expected; thus began a healthy mix of collaboration between theorists and experimenters.