ABSTRACT
This chapter deals with the radiation properties of longwave electromagnetic impulses generated by lightning within the waveguide between the Earth's surface and the ionospheric D layer and lower E region (atmospheric waveguide). It considers the transfer function of the atmospheric waveguide. The physics of electromagnetic wave propagation within the channel must thus be derived from full wave theory, because the ray concept breaks down in this case. The transfer function of the atmospheric waveguide in thus describes the deviation from ideal propagation conditions. Transverse electric fields (TE waves: horizontal electric field component orthogonal to the direction of propagation) are only of minor importance for longwave sferics. The atmospheric waveguide therefore has two windows, one in the Extremely low frequency (ELF) range and the other one in the very low frequency range. No exact theory of mode propagation in the real atmospheric waveguide presently exists, which accounts for horizontal inhomogeneities and the real geomagnetic dipole field.
