Abstract
The belief that it may be possible to interpret geomagnetic observations in terms of upper atmospheric winds is based on the theory proposed by Balfour Stewart in 1882 to account for the observed daily magnetic variations. Balfour Stewart's suggestion was that the upper atmosphere is highly conducting and that this conducting air in its daily horizontal tidal movement across the vertical component of the earth's field induces by dynamo action a varying electric current whose magnetic field is observed at the ground as the magnetic daily variation. The theory of the magnetic solar daily variation, and of a corresponding lunar effect, was developed by Schuster and later by Chapman. All the problems of the theory have not been resolved but its essential correctness has received support in three ways which have led to its general acceptance : the experimental discovery of the ionosphere; characteristics of the magnetic effect which accompanies a large solar flare; and the discovery by a rocket ascent in 1949 of a discontinuity of the geomagnetic field at about 100 km due to the concentration of an electric current there.
The atmospheric dynamo theory of the magnetic daily variations implies that any steady wind in the upper atmosphere must also be reflected in the magnetic records, and during the past ten years Wulf has examined this possibility, with limited success. Recently Vestine, proceeding on the assumption that all currents causing magnetic disturbances are located within the high atmosphere, has derived wind systems compatible with the annual variation of magnetic disturbance and with the features shown by a particular magnetic storm. Although the validity of Vestine's basic assumption is in considerable doubt, his contribution remains an important one and points the way to future progress. The prospect of obtaining actual measurements of the ionospheric winds by means of the newly developed radarsonde, combined with the regular ionospheric measurements now available, make it probable that important progress with this problem will be made in the next few years.