Abstract
The brief period between the conception of the interplanetary magnetic field and conclusive proof of its existence has been an exciting one. Imaginative theoretical developments and careful experimental verification have both been essential to rapid progress. From the various lines of evidence described here it is clear that an interplanetary magnetic field is always present, drawn out from the sun by the radially streaming solar wind. The field is stretched into a spiral pattern by the sun's rotation. The field appears to consist of relatively narrow filaments, the fields of adjacent filaments having opposite directions. At the earth's orbit the field points slightly below the ecliptic plane. The magnitude of the field is steady and near 5 gammas in quiet times, but it may rise to higher values at times of higher solar activity. A collision-free shock front is formed in the plasma flow around the earth. In the transition region between the shock front and the magnetopause the magnitude of the field is somewhat higher than it is in the interplanetary region, and large fluctuations in magnitude and direction are common. A shock front has also been observed in space between a slowly moving body of plasma and a faster, overtaking plasma stream.
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