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

地磁变化场的内部项常被用来估计地球上的电导率分布。然而，海洋的电导率(4S/m)远大于固体地球的浅部电导率(10^&lt；-2&gt；-10^&gt；-4&gt；S/m)，因此，由外场变化引起的海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，但地磁太阳静日变场(Sq)和称为DST或DP的小于几天周期的地磁扰动场受到海洋洋流的影响，因为海洋中诱导的洋流的时间常数约为10小时。结果表明，DST场的电流比同期的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，而这些电流在很大程度上受到大陆的阻挡。其次，得到了日本周围感应电流的更精细的分布。该方法考虑了详细的海陆分布，自洽地处理了自感效应，并用全局计算的感应电流值作为边界导电值。仅由海陆分布估计的合成感应场将被用作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.