Stimulated Second Harmonic Emissions During Ionospheric Heating Experiments

Stimulated Electromagnetic Emission (SEE) is secondary radiation produced during experiments involving the interaction of high power high frequency (HF) radio waves with the ionosphere. SEE has been developed into a useful remote sensing tool for the modified ionosphere with potential for aeronomy studies over the past four decades. Early experiments primarily considered emissions within a bandwidth of approximately 100 kHz of the pump (transmit) frequency ω
0
, so-called wideband SEE (WSEE), but work over the past decade has concentrated more on SEE within 100 Hz of the pump frequency, so-called narrowband SEE (NSEE). More recent investigations over the past few years carried out at the High Frequency Active Auroral Research facility (HAARP) in Alaska, have considered Second Harmonic Generation (SHG) in the narrowband SEE spectrum around twice the pump frequency 2ω
0
. This presentation will provide a review of the recent experimental observations of SHG at HAARP as well as some basic concepts of the nonlinear wave-wave processes that have been proposed to produce the observed SHG. A focus will be placed on the potential of SHG for diagnostic information during ionospheric heating experiments. It is useful to note that SHG has been developed into a useful diagnostic tool in the Laser Plasma Interaction (LPI) community since the 1970’s and this has attracted considerable interest in experiments, theory, and modeling. This knowledge base may possibly be applied to SHG observed during ionospheric heating experiments. However, for ionospheric interactions with high power radio waves, the earth’s magnetic field is found to play an important role in the spectral characteristics of the observed SHG spectrum as will be discussed in the presentation. As will be shown, the spectral lines are dependent on both the proximity of the pump frequency to a harmonic of the electron gyrofrequency Ω
ce
as well as the transmit antenna beam angle orientation relative the geomagnetic field. It will be shown that the SHG spectrum is easily as richly structured as the spectrum near the pump frequency as shown in Figure 1. These spectral lines are proposed to be produced by wave-wave interactions involving low frequency ion wave modes in the ionospheric plasma. These may include ion acoustic IA as well as ion Bernstein (IB) modes. The latter wave modes being ordered by the ion gyrofrequency Ω
ci
which is roughly 50Hz in the ionospheric interaction region. The correlation of the spectral lines at ω
0
and 2ω
0
as well as possible difference in the temporal evolution during heating underscores the importance of SHG as a potential important diagnostic tool during ionospheric heating experiments.