Ground-based transmission of high-power high frequency (HF) radio waves into the Earth’s ionosphere produces a plethora of physical phenomena that result from complex nonlinear processes. This field has commonly been referred to as ionospheric heating or ionospheric modification in pertinent literature, and it has a long, rich history dating back at least to the 1960s. The phenomenon induced by the interaction of a radio wave and ionospheric plasma may serve as a laboratory for investigating fundamental phenomena as well as facilitate ionospheric diagnostics. This chapter will concentrate on a specific phenomenon which is stimulated secondary radiation that exists in the frequency band typically within about 1 KHz of the transmit frequency. This narrowband stimulated radiation has been studied extensively over the past decade or so at the High Frequency Active Auroral Research Program (HAARP) Facility in Gakona, Alaska. HAARP’s unprecedented capabilities in terms of maximum power, transmit beam and frequency agility, have resulted in discovery of several new nonlinear ionospheric phenomena that may be leveraged for new diagnostics by using the narrowband stimulated radiation spectrum. This chapter will provide a synopsis of the most recent observations, theory, and advanced computational modeling and provide a future outlook on possibilities for future avenues of investigation.
Part of the book: Ionosphere