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The first report is made of a holistic investigation of stimulated electromagnetic emission (SEE) during ionospheric electron gyroharmonic heating using a diagnostic approach at the High Frequency Active Auroral Research Program facility. The evolution of SEE polarization and plasma irregularity development near the third electron gyroharmonic 3f(ce) is investigated, which provides new insights into SEE generation mechanisms by the associated parametric decay instabilities. New more complex SEE spectral line behavior is clearly observed for varying transmitter beam angles. New SEE spectral emissions are observed at similar to 75kHz when pumping below 3f(ce). The high-frequency radar echoes and SEE polarimetry appear asymmetric for pumping above 3f(ce), which implies that the broad upshifted maximum spectral line formation involves plasma irregularities scattered by high-frequency radar echoes. This has important implications for understanding artificial ionization layer generation during ionospheric modification.

Plain Language Summary Nonlinear interaction of high-power electromagnetic waves and magnetized plasmas produces a plethora of fundamental phenomena that can be exploited for powerful diagnostics of the Earth's ionospheric plasma. Stimulated electromagnetic emissions (SEEs) and plasma density turbulence, observable with high-frequency radars, arise from this nonlinear interaction as well as artificial ionization layers. The later has received considerable attention recently due to the importance of the associated fundamental physics and chemistry processes as well as the implications for modern communication and navigation systems. Artificial ionization layers are still not well understood, and this work seeks to provide insight into the generation mechanism by reporting the first results of a novel diagnostic approach that utilizes SEE and high-frequency radar measurements. The experiments were performed at the High Frequency Active Auroral Research Program facility. The evolution of SEE polarimetry and turbulent plasma density structures exhibits a prominent asymmetry when sweeping the transmitter frequency near electron gyrohamonic frequencies. A direct connection was observed between the plasma density structures and SEE spectral lines known to be associated with ionization layers. The observed asymmetric behavior therefore provides new insights into the linkage between ionization layers and generation of plasma density structures with specific characteristics.