Figure 1. (a) Averaged variations in ELF-VLF emission intensity in the range from 0.5 to 10 kHz during SID. (b) Curves given in Figure 1a are extrapolated to the frequency range above 10 kHz by using the data of Rizzo Piazza and Kauffman [1975] on the frequency of transition from the VLF signal weakening to enhancement.