抄録
Harmonic analyses have been made of the geomagnetic horizontal component (H) and the vertical component (Z) from several observatories. These analyses indicate variations of tens of nanoteslas in the period range 13 to 30 years. The largest variation occurred for periods of approximately 25 years. For the pass band 13 to 30 years, several cases were found in which a group of neighboring observatories displayed similar variations. It is quite clear that the origin of these signals must be found in the core of the earth rather than outside the earth. The variations can not be explained by the westward drift of a core dynamo with a detailed fine structure. Hydromagnetic waves in the core offer a better explanation.
