Stimulated emission and radar observations of rf-ionosphere interaction experiments

High power HF radiowaves launched from ground-based transmitters interact with overdense ionospheric plasma to produce strong nonlinear effects. Where available, VHF/UHF radar data reveal several spatial and temporal signatures in the plasma line backscatter indicative of strong turbulence effects at many scales. Stimulated electromagnetic emissions (SEE) which propagate to HF receivers on the ground may provide information complementary to plasma line backscatter. Many experiments using SEE receivers have been performed using relatively long (seconds) HF pulses. Long HF pulses are sufficient to produce irregularities and turbulence over several spatial and temporal scales, incorporating many effects, evident in the complex SEE spectra recorded. We report a series of experiments performed at HAARP (Gakona, Alaska) using both HF radar and SEE diagnostics together with a range of HF pulse widths down to ten mil-liseconds. Using short HF pulses, we are able to discriminate, characterize, and compare prompt SEE spectra over the time-scales of typical radar features. HF radar studies of HF pump-induced field-aligned irregularities allow the formation, evolution, and transport of these intermediate-scale structures to be characterized. Results of numerical modeling of the development of strong short-scale turbulence and mesoscale irregularities are presented for comparison with these and similar experiments.