Nonlinear Phenomena Arising from Radio Wave Heating of the Lower Ionosphere.

Abstract : This document describes a theoretical and experimental study of the interaction of high power, high frequency radio waves with the lower ionosphere. The theoretical calculations presented here show that the electron temperature of the ionospheric plasma can be greatly enhanced when the plasma is irradiated by a powerful groundbased HF transmitter with an effective radiated power of the order of 100 MW. If this plasma heating is maintained for times exceeding a few seconds, the composition of the plasma can also be altered. These temperature and composition modifications cause significant changes in the plasma conductivity and wave absorption in the medium. Two experiments were conducted in order to test for the predicted absorption and conductivity modifications: a vertical incidence plus absorption experiment and a nonlinear demodulation experiment. Data from the absorption experiment clearly show a large (9 dB) increase in wave absorption at 2.4 MHz due to a high power (60 MW ERP) HF heating of the ionosphere. The nonlinear demodulation experiment generated strong VLF radiation when the ionosphere was irradiated by a powerful modulated HF wave. These VLF signals are believed to be due to HF heating induced conductivity modulation of the dynamo current system. (Author)