RESONANCES IN THE GENERATION OF ELECTROMAGNETIC SIGNALS DUE TO THE THERMAL CUBIC NONLINEARITY IN THE LOWER IONOSPHERE

The resonances in generation of electromagnetic waves due to the therm al cubic nonlinearity are investigated theoretically for a broad frequ ency range. The possibility of an experimental check of the effect is discussed. The experimental results for one of the resonances in the V LF frequency band are presented. The magnitude of the ionospheric cond uctivity modulation produced at VLF by HF heating using the thermal cu bic nonlinearity mechanism is found to be of the same order as that pr oduced using direct amplitude modulation of the HF waves, for VLF sign als between 1 to 5 kHz; Above 10 kHz VLF wave generation using the cub ic nonlinearity mechanism is found to be more efficient. (C) 1998 Else vier Science Ltd. All rights reserved.