The annual variation of the night-time values of the geomagnetic field.

The authors present the results of an analysis of the variation of period of one year in night-time H data. The analysis indicates that this annual variation comprises of two components. (1) A component whose amplitude depends on latitude and is of opposite phase in the two hemispheres. This component is assumed to be due to the change of the latitude of the ring current caused by the solar wind, as originally proposed by Malin & Isikara (1976). (2) A smaller component which, in a given year, has an amplitude and phase that is approximately the same at all latitudes but which may change from year to year. Its origin is not known but it is suggested that it may be found in the modulation of a distant current, such as the cross-tail current of the night-time magnetosphere.