First 100 ms of HF modification at Tromsø, Norway

Experiments were performed with the high‐power, high‐frequency (HF) facility at Tromsø, Norway to test theoretical predictions for the excitation of ion and Langmuir oscillations in the ionosphere. The principal diagnostic of wave‐plasma interactions was the VHF radar at the European Incoherent Scatter (EISCAT) facility. This radar is collocated with the HF facility. High‐resolution radar techniques were used to monitor the temporal development of the ion and Langmuir oscillations. HF pulses 100 ms in duration were periodically transmitted into a smooth background F region plasma. Measurements of the radar backscatter spectra show that all key spectral features predicted by strong Langmuir turbulence theory are simultaneously present in the plasma and that their evolution is in agreement with theoretical expectations. However, several new features have been observed that are not anticipated by current theory. The experimental results reinforce the notion that new theoretical developments are needed to accommodate the large HF electric fields produced at Tromsø and treat the electron acceleration process in a self‐consistent fashion.