Abstract
Sakai et al.(1)(3)(8) demonstrated that a strong magnetic field at a lightning strike induces isothermal remanent magnetization in the materials around the point of lightning strike. In some cases, the ground or underground materials, or both are possibly heated by the lightning strike. They may thereby, acquire another type of magnetization, called thermal remanent magnetization. Fulgurite that is produced underground at the lightning is a typical result of the heating effect caused by the lightning electric current. We investigated the remanent magnetization of fulgurite found at Iwamizawa, Hokkaido, Japan. The study verified that the fulgurite has thermal remanent magnetization which recorded the geomagnetic field. We estimate that the temperature of the fulgurite immediately after the lightning strike was over Curie temperature (580°C) of the included magnetite minerals. Experimental results indicate that when the fulgurite was formed, the lightning current already disappeared and did not affect the remanent magnetization. A magnetic study at the lightning-damaged area could investigate the heating effect of lightning current as well as its path and magnitude.
