Research on electric conductivity in the Earth considering induced currents in the Ocean

地磁变场的内部项常被用来估计地球的电导率分布。然而，海洋的电导率（4S/m）远大于固体地球浅部的电导率（10^<-2>-10^<-4>S/m），因此外场变化引起的电流受海洋分布的影响很大。当然，这种影响既取决于外场的空间尺度，也取决于外场的时间尺度，但由于海洋中诱导的海流的时间常数约为10小时，因此，周期小于几天的地磁太阳平静日变化场（Sq）和地磁扰动场（Dst或Dp）受到海洋中海流的影响。本文利用球谐法模拟了Sq场和Dp、Dst场在1小时至数天期间在海洋中诱发的第一次海流，并估计了这些海流在若干观测站引起的地磁场变化。结果表明，在同一时期，Dst场的电流比Sq场和Dp场的电流弱。这是因为Dst磁场需要纬向流动的感应电流，而这种电流在很大程度上被大陆阻挡了。其次，得到了日本周围更精细的感应电流分布。该方法详细考虑了海陆分布，自洽地处理了自感的影响，采用全局计算的感应电流值作为边界导通。仅用海陆分布估计所得的感应场作为电导率异常分布的标准。

Internal terms of the geomagnetic variation fields are often used to estimate the electrical conductivity distribution in the Earth. However, conductivity of the ocean (4S/m) is much larger than that of the shallow part of the solid Earth (10^<-2>-10^<-4>S/m) , and thus currents induced by the variation of the external fields are much affected by the distribution of the ocean. Of cource, the effects depend on both the spatial and time scale of the external field, but geomagnetic solar quiet daily variation fieid (Sq) and geomagnetic disturbance field called Dst or Dp of less than a few days period are affected by the currents in the ocean because time constant of the currents induced in the ocean is about 10 hours.In the present study, first currents induced in the ocean by the Sq field and Dp and Dst fields of the periods from one hour to a few days were globally simulated using spherical harmonics, and geomagnetic field variation at several observatories by the currents were estimated. It was found that the currents for the Dst field are weaker than those for the Sq or Dp fields of the same period. This is because the Dst field requires zonally flowing induced currents which are largely obstructed by the continents. Next, finer distribution of the induced currents around Japan was obtained. In the method detailed sea-land distribution is considered and the effects of the self inductance is treated self-consistently, and the values of the induced currents calculated globally are used as the boundary conduction. The resultant induced field estimated by only land-sea distribution will be used as a standard for anomalous distribution of the conductivity.