ELF VLF RADIO SIGNALS CAUSED BY IONOSPHERIC DEMODULATION OF MF/HF RADIO TRANSMITTER SIGNALS/

A quantitative theory of the generation of ELF/VLF signals, which are caused by the ionospheric demodulation of modulated signals from broad casting MF/HF transmitters, is developed. Two cases are considered whe n the ionospheric current, modulated by broadcasting signals, is a one -dimensional linear current (the polar electrojet), and when it is an extended two-dimensional current sheet. It is shown that the main demo dulation effect is caused by the formation of an ionospheric travellin g wave antenna. In the case of the polar electrojet, this antenna radi ates ELF/VLF waves in two directions, one of which coincides with the line connecting the broadcasting transmitter with the ELF/VLF receiver , and the other one is in a direction which is the mirror image of thi s line relative to the current direction. In this case the polarisatio n of the ELF/VLF magnetic field component varies widely. In the case o f the extended ionospheric current, the demodulation takes place along the line connecting the MF/HF transmitter with the ELF/VLF receiver, and the horizontal ELF/VLF magnetic field component is normal to this line. Amplitude and phase distortions in the ELF/VLF signal are absent below a frequency less than or similar to 1kHz and appear above simil ar to 1kHz. Quantitative estimates of the amplitude of the ELF/VLF sig nal give a value similar to 0.15pT which is close to the experimentall y observed value. (C) 1997 Elsevier Science Ltd.