Abstract
The annual variation of the geomagnetic field is a phenomenon resulting from the movement of the mean latitude of the ring current between summer and winter and is of importance for the determination of deep-mantle conductivity. However, if the annual term is determined from data based on all-day means, it is found to include terms arising from the seasonal variation and modulation of Sq. This effect is investigated both theoretically and experimentally, and it is concluded that the annual variation is best determined from night-time data.
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