Geomagnetic variation anomaly on Hachijo-jima Island.

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.