抄録
The magnetic field changes due to cloud-to-ground strokes have been measured by loop antennas, which have been installed in the north-south direction and the east-west direction, respectively, in winter and summer seasons during 1982 through 1989. In consequence of the measurements, it is found that there is difference between the magnetic field change due to summer lightning and that due to winter lightning, that is, the magnetic field change measured by the N-S loop antenna is the same as the magnetic field change measured by the E-W loop antenna in summer lightning, but not the same in winter lightning (Winter ligtning in this paper is the triggered lightning initiated at the top of a steel tower of a transmission line.). It is considered that the difference between the magnetic field change measured by the N-S loop antenna and that by the E-W loop antenna in winter lightning is caused by the anomalous shape of the lightning channel, such as the large tortuosity of the channel close to the ground. The effects of the tortuosity of the channel on the magnetic field change is shown by the numerical analysis of magnetic field change due to the lightning stroke current, which propagates along the tortuous channel. When the lightning stroke current is estimated from the magnetic field change, it should be noted that the shape of the channel has an important effect on the magnetic field change.
