A search for the location of the HF excitation of enhanced ion acoustic and Langmuir waves with EISCAT and the Tromso heater

In an effort to understand the mechanisms which give rise to the enhanced ion acoustic and Langmuir waves in HF modification experiments, measurements were made with the EISCAT Tromso heater and the two incoherent scatter radars to locate the regions of enhancement in space. Simultaneous measurements were made of the enhancements with both the VHF (224 MHz) and the UHF (933 MHz) radars, the latter being scanned in the magnetic meridian plane between vertical and field aligned through the Spitze angle. The results show that enhanced bottom-side and topside ion acoustic and plasma waves at UHF and VHF are a common phenomena in these data. UHF topside enhanced plasma waves were not observed, possibly due to lack of radar sensitivity. At UHF the enhanced ion waves were strongest when the radar was pointed between the Spitze angle and geomagnetic field aligned direction. The topside features we believe give evidence for the production and propagation of Z-mode waves during this experiment. There is no evidence in these data for a decay of Z-mode waves into Bernstein and lower hybrid waves inside the overdense F layer as we believe to have observed previously.