ELF radio signals in the auroral ionosphere generated by non-linear demodulation of LF and/or MF transmissions
We have conducted experiments measuring ELF and VLF radio signals in the afternoon to early morning (local time) between 24 March and 4 April 1979 in Northern Scandinavia. Apart from those of a natural origin (atmospherics, chorus, and so on), we received timing signals, six pips which occurred on the hour UT; the duration of each pip, of frequency 1 kHz±0.5 Hz, was 105±8 ms. A spectrogram of these is shown in Fig. 1. We have established that these are not due to non-linear demodulation of strong higher frequency radio signals in any part of the equipment. We consider that they are produced by non-linear demodulation of signals from two or more amplitude modulated transmitters in the USSR operating at 173, 200, 236, 263 and 657kHz. The demodulation occurs only during enhanced auroral electrojet activity.