At the lightning, a strong magnetic field is induced around the path of the pulse current between the cloud and the ground. We investigated the remanent magnetization of the soil and ground material most likely acquired under this magnetic field. Both clockwise and counterclockwise rotating remanent magnetizations were found, which are believed to be corresponding to the positive and negative polarities of lightning current. The remanent magnetization can be a fossil of lightning current preserved in

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-Microwave multiple radio relay stations are often built on mountains, and the stations are susceptible to damage from lightning. Therefore, it is important to take adequate lightning protection measures to ensure that communication devices are not damaged by any lightning surge current that penetrates from a lightning rod fitted on such stations. In most cases, the penetration route of the lightning surge current is uncertain, and it is difficult to specifically evaluate the effectiveness of measures against lightning. We calculated the branch aspect of lightning surge current in actual microwave relay stations using the finite difference time domain (FDTD) method, which is one way to numerically analyze electromagnetic fields, to directly solve Maxwell's equations. By comparing the calculated results with measured results obtained by injecting a pulse current into a microwave relay station, we verified that the current peak value of the calculated results corresponded with the measured results well, both when a steel tower was located on the ground and when it was located on the roof of a microwave relay station. We confirmed that the FDTD method can be used to understand the branch current of lightning surges and to study lightning protection measures at microwave relay stations.

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