抄録
The technique of complex demodulation is applied to low latitude geomagnetic field data to isolate the long term changes in the amplitude and phase of the semiannual and annual variations. The phase for semiannual variation is in close agreement with that anticipated from equinoctial hypothesis. The amplitude does not show significant dependence on the 11-yr solar cycle. Comparable time variation for all the hours of the day leads to the conclusion that the semiannual variation originates in the disturbance field. A regular oscillation with a 6-7 year periodicity and a long-term variation of the amplitude of annual oscillation corresponding to 14 LT appear as new results.
