Abstract
A new spectoscope recording continuously the amplitude-frequency spectrum of VLF atmspherics has been developed. The receiver sweeps the frequency giving complete coverage of the band of 5-70kc/s repeatedly and the output is displayed on an intensity modulated cathode-ray tube, which is photographed on a slowly moving film.
Observations have been carried out and it appears that the result provides an excellent experimental base for the mode theory of VLF ionospheric propagation. It is found that the frequency-spectrum of distant atmospherics shows a broad intensity maximum around 10kc/s and decreases its intensity towards higher frequencies with undulating peaks. Marked selective absorption bands appeared in the spectrum are variable according to the time of day, and they might be assoiated with cut-off frequencies of the waveguide bounded by the earth and the ionosphere. The solar flare effect on VLF atmospherics propagation is also revealed, which indicates a sudden shift of the spectrum to higher frequencies owing to the increase of ionization and the lowering of a reflecting height of the ionosphere.
