13. A Theory of Frequency-Dependent Effect of an Island on Geomagnetic Variations

Abstract

Self-induction effect of electric current flow around a circular hole in a thin conducting sheet is studied in an approximate way. It turns out, however, that the self-induction is not so large that marked frequency dependence of the vertical field component as observed, for example, on Hachijo-shima Island can be attributed to this origin. A phase lag amounting to several degrees seems likely to have arisen by the self-induction effect provided a periodic variation having a period of 30min. and an island of which the diameter is 10km are assumed.|離島上で観測される地磁気短周期変化が,いわゆる離島効果を示すことは,最近の観測によつて確立された.磁場が北向きに変化する場合には,鉛直成分対水平成分比(⊿Z/⊿H)は通常島の北端で負,南端で正となる.しかし,たとえば八丈島の観測点でみられるように,⊿Z/⊿Hが変化の周期によつて変化し,その符号が変化する場合もある.従来の理論では自己感応の影響が省略されていたが,この論文では,近似的に自己感応の影響を議論し,⊿Z/⊿Hの周期依存性が説明できるか否かをみた.その結果自己感応の影響は比較的小さく,そのような依存性は説明不可能で,たとえば周期によつて海中に流れる電流の場所が異なるというような原因を考えざるを得ないという結果が得られた