Semiannual Variation of Geomagnetic Disturbance and Its Modulation of Shorter Period Variations

By making use of the techniques of power spectrum analysis and simple linear filtering procedures, it is shown that the amplitude of the 27-day variation of the geomagnetic disturbance index Ci is subject to a significant semiannual modulation. It is also shown that there is no amplitude modulation of this component with other periods such as the annual or 11-year period. The semiannual amplitude modulation, however, is also found in variations of Ci for time periods between 27 days and 6 months, and the phase of the modulation is closely locked in phase with that of the 6-month variation of Ci itself. This close agreement between the 6-month variation of Ci and the 6-month amplitude modulation of such components as the 27-day variation forms the basis of a test between the two principal hypotheses that have been proposed to explain the 6-month variation of geomagnetic disturbance; namely, the so-called 'axial' and 'equinoctial' hypotheses. In the axial hypothesis, the important element is the inclination of the sun's axis of rotation to the ecliptic plane, whereas the equinoctial hypothesis depends upon the inclination to this plane of the earth's axis of rotation and its associated dipole magnetic field. The result of the test clearly favors the equinoctial hypothesis.