Modal phenomena in the natural electromagnetic spectrum below 5 kHz

This paper presents new measurements of the magnetic field component of nat urally produced electromagnetic radiation in the ELF/VLF range. The measure ments are compared to calculations based on modal propagation theory. The n octurnal spectrum below 5 kHz often contains a sharp increase in spectral l evel at the cutoff frequency of the first mode (similar to 1700 Hz) and a d istinct variation of the spectral behavior at the cutoff frequency of the s econd mode (similar to 3400 Hz), where the indicated cutoff frequencies wer e calculated for perfectly conducting Earth and ionosphere, with the ionosp here 88 km above the Earth. These features are attributed to an enhancement in the first (quasi-transverse electric (QTE(1))) and second (QTE(2)) prop agating modes at night, which makes their level comparable to the basic (tr ansverse electromagnetic) mode. Another spectral feature, which also prevai ls during the night, is semiperiodic fluctuations of the spectrum between t he two cutoff frequencies, with a short period at the low end of the band a nd a gradual increase of the period as frequency increases. A similar semip eriodic fluctuation is apparent above the cutoff frequency of the second mo de up to the limit of the measured band at 5 kHz. These semiperiodic fluctu ations of the spectrum are related to the modal nature of the electromagnet ic wave propagation. We show that computations of the spectrum of the ELF/V LF radiation emitted by a vertical lightning in the Earth-ionosphere wavegu ide gives spectra that compare well with the measurements.