Excitation of Sidebands Due to Nonlinear Coupling Between a VLF Transmitter Signal and a Natural ELF Emission

Abstract : Symmetric sidebands are observed in the ionosphere by the AUREOL 3 satellite when it passes at a height of 1200 km above the VLF transmitter at the Komsomolsk‐on‐Amur Alpha station (50°5 N, 135° E, frequency 11.90 and 12.65 kHz). The sidebands are about 500 Hz off the carrier frequency of Alpha pulses. They are approximately 20 dB lower than the transmitter signal, and they appear only when ELF natural emission above the local proton gyrofrequency is observed. The data are presented and analyzed. The nonlinear coupling of the VLF transmitter signal to natural ELF emission is invoked to explain the symmetric sidebands. It is shown that the nonlinear current excited by the beats of VLF and ELF waves is strong enough to explain the sideband amplitude.