Geomagnetic variation anomaly on Hachijo-jima Island.

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Abstract
Time-varying magnetic fields were recorded during the winter of 1972 with an array of 5 flux-gate magnetometers on Hachijo-jima Island. The observed records show a typical island effect, e. g. a reversal of sign of the vertical magnetic fields between the northern and southern observation points on the island when the inducing field varies in the north-south direction. The period dependence of the anomaly is examined for a period range from 5 to 240min by making use of the transfer function technique. A uniform period dependence is seen throughout the stations for periods longer than 30min or so. As a result it is shown that the geomagnetic variation anomaly on Hachijo-jima Island is accounted for by the central Japan anomaly superposed on the island effect, and the strong period dependence is due to the central Japan anomaly. A numerical calculation of induced electric currents and magnetic fields is made on the basis of a non-uniform thin sheet model representing the island. As for transfer function Au, the calculated values are about twice as large as the observed ones as is the case for Miyake-jima Island (HONKURA, 1971). It is concluded, therefore, that a highly conducting layer must lie very close to the earth's surface underneath Hachijo-jima Island.

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