Temporal Evolution of Second Harmonic Narrowband Stimulated Electromagnetic Emissions

Abstract:

Secondary radiation produced during the interaction of high power high frequency (HF) electromagnetic waves with the ionosphere, known as stimulated electromagnetic emissions (SEEs), have through experimental, theoretical and computer modelling studies evolved into a powerful diagnostic of the modified ionosphere yielding information such as electron temperature, ion mass, turbulence evolution etc. Although there have been comprehensive studies of SEEs near the pump wave frequency, it is fairly recently that interest has renewed in the generation of SEEs near the second harmonic of the pump wave frequency, a process known as second harmonic generation (SHG), to build upon the few past studies on SHG. The initial goal is to systematically study SHG via experiments/computer modeling and understand the causation of the SEEs that result from SHG. The motivation is to leverage the well-established diagnostic capabilities of SHG in the field of Laser Plasma Interactions to possibly develop similar diagnostics of the modified ionosphere. In this presentation, results from two separate experiments, which were conducted at the High Frequency Active Auroral Research Program (HAARP) facility, Gakona AK are reported. In both experiments, the pump frequency was stepped near the second harmonic of the electron gyrofrequency and the transmit beam aligned along the local geomagnetic field. In one experiment, the pump power was maintained at the maximum available transmit power of 2.80 MW whilst in the second experiment, the power was linearly ramped from 3% of the maximum available power to the maximum available power of 2.80 MW. Highlights of the observations in the SEE spectra are SEEs ordered by the oxygen ion gyrofrequency, the dominant species in the F-region ionosphere, within ±1 kHz of both the pump frequency and second harmonic of the pump frequency. The SEEs are attributed to a parametric decay instability (PDI) involving electron and ion Bernstein waves. The time evolution of the SEEs show interesting features including growth, saturation and decay.