From AMS Glossary
The hypothesis, first proposed by Balfour Stewart, that explains the regular daily variations in the earth's magnetic field in terms of electrical currents in the lower ionosphere, generated by tidal motions of the ionized air across the earth's magnetic field.
Whitten, R. C., and I. G. Poppoff 1971. Fundamentals of Aeronomy. Wiley & Sons, New York, . p. 219.