Excitation of Earth-ionosphere cavity resonances by lightning flashes


  • E. T. Pierce


The existence of resonances at frequencies below 100 cps was originally predicted by Schumann. Their presence has since been experimentally confirmed. In subsequent theoretical development it has largely been assumed that the excitation is due to the return stroke of a flash to earth. Observations in the VLF band (3 to 30 kc/s) have been extrapolated to the frequency range of less than 100 cps. The limitations of this approach are discussed. Simple models are developed on an empirical basis for the temporal variation of electrostatic moment during cloud discharges and flashes to earth. Differentiation and Fourier methods yield the spectral content of the radiating signals exciting the resonances. It is shown that cloud and ground flashes are of comparable effectiveness in the excitation of the lower-frequency resonances. The implications of this conclusion are examined.