On the annual variation of geomagnetic east component at a low latitude station

Results of spectral analysis show that the amplitude and phase of annual modulation of the east (Y) component of the geomagnetic field exhibit large local time variation. The diurnal variations of amplitudes as well as phase angles are consistent with that expected from the seasonal modulation of diurnal variations associated with Sq and SD. The annual modulation of SD is considerably smaller than the Sq modulation but is in phase with Sq in the morning hours and plays an important role in determining the local time and magnetic activity dependence of the amplitude of the annual term in the absolute Y-field.