Narrowband Stimulated Radiation During Ionospheric Heating Experiments: Recent Observations, Theory, and Modeling

Ground-based transmission of high-power high frequency (HF) radio waves into the Earth’s ionosphere produces a plethora of physical phenomena resulting 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. Phenomena induced by the interaction of radio waves and the ionospheric plasma may serve as a laboratory for investigating fundamental electromagnetic wave phenomena as well as facilitate diagnostics for the near-earth space environment aeronomy. Although important phenomena produced during ionospheric heating include production of geo-magnetic field aligned density irregularities, optical emissions, and production of plasma turbulence, this presentation will primarily concentrate on a specific phenomenon which is stimulated secondary radiation that exists in the frequency band typically within roughly 1kHz of the transmit frequency ω
0
. As will be discussed, the spectrum of this radiation may be used for diagnosing the state of the heated plasma volume by utilizing frequency sidebands, produced by nonlinear processes, shifted above and below ω
0
. Figure 1 shows a typical spectrum of secondary radiation around ω
0
and 2ω
0
[1].