Geomagnetic Variation Anomaly on Miyake-jima Island

Abstract

Simultaneous observations of the three components of geomagnetic variations were made with five flux-gate magnetometers and two Askania magnetometers on Miyake-jima Island during a period from November, 1969 to July, 1970. Observed records are analyzed mainly by making use of the techniques of power spectral analysis and transfer function. The island effect on geomagnetic variations is approximately separated from the effect of the central Japan anomaly by comparing transfer functions determined at each observation point. As a result of the analysis, it is found out that the central Japan anomaly gives rise to peculiar frequency characteristics at stations on Miyake-jima Island.As for the island effect, a model calculation is made on the basis of a non-uniform thin sheet model. According to the comparison between the observed and calculated transfer functions, it turns out that the observed values amount to only about fifty percent of calculated ones as regards the transfer function Au. But in the case of Bu, the observed values seem to agree with the halves of calculated value as in the case of Au provided a constant, 0.1 say, is subtracted from the observed values. The constant thus introduced may be attributed to the effect of an east-west undulation of the conductor in the upper mantle near the Izu-Mariana island arc. The fact that the observed values amount to only about fifty percent of the calculated values seems to reflect the coupling between the sea water and the subterranean conductive layer which weakens the current induced in the sea.