where wpe, wce are the plasma and electron cyclotron frequencies, respectively, e is the fundamental charge, me is the electron mass, and e0 is the permittivity of free space. Therefore, the rotation angle of polarization of

en e

Magnetic Flux Density B

r Linearly Polarized Beam

Fig. 1. Faraday effect.

the beam propagated at distance L(= L\ — L2) is obtained as follows:

where A is the wavelength of the propagating beam. Since 5 is proportional to A2, the rotation angle for visible light is small. Therefore, the polarization angle rotation must be measured with high accuracy in order to detect lightning discharge.

When the Faraday effect is applied to lightning measurement, the atmosphere needs to be partially ionized, and the magnetic flux due to the lightning discharge must exist. Cloud-to-cloud discharge, which causes 20-30 times continuous discharge, satisfies those conditions.

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