This paper presents new measurements of the magnetic field component of naturally produced electromagnetic radiation in the ELF/VLF range. The measurements are compared to calculations based on modal propagation theory. The nocturnal spectrum below 5 kHz often contains a sharp increase in spectral level at the cutoff frequency of the first mode (∼1700 Hz) and a distinct variation of the spectral behavior at the cutoff frequency of the second mode (∼3400 Hz), where the indicated cutoff frequencies were calculated for perfectly conducting Earth and ionosphere, with the ionosphere 88 km above the Earth. These features are attributed to an enhancement in the first (quasi-transverse electric (QTE1)) and second (QTE2) propagating modes at night, which makes their level comparable to the basic (transverse electromagnetic) mode. Another spectral feature, which also prevails during the night, is semiperiodic fluctuations of the spectrum between the two cutoff frequencies, with a short period at the low end of the band and a gradual increase of the period as frequency increases. A similar semiperiodic fluctuation is apparent above the cutoff frequency of the second mode up to the limit of the measured band at 5 kHz. These semiperiodic fluctuations of the spectrum are related to the modal nature of the electromagnetic wave propagation. We show that computations of the spectrum of the ELF/VLF radiation emitted by a vertical lightning in the Earth-ionosphere waveguide gives spectra that compare well with the measurements.
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