F. T. Djuth¹, B. Isham^2,5, M. T. Rietveld^3,5, J. H. Elder¹, T. Hagfors³, C. La Hoz⁴

¹Geospace Research, Inc., 550 N. Continental Boulevard, El Segundo, California
²Interamerican University, Bayamón, Puerto Rico
³Max-Planck Institut für Aeronomie, Katlenburg-Lindau, Germany
⁴Auroral Observatory, University of Tromsø, Tromsø, Norway
⁵Also at the European Incoherent Scatter Facility, Ramfjordmoen, Ramfjordbotn, Norway

In June 1998, experiments were performed with the high-power, high-frequency (HF) facility at Tromsø, Norway to test competing theories for the excitation of ion and Langmuir oscillations in the ionosphere. The principal diagnostic of wave-plasma interactions was the VHF (224 MHz) 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. An HF pulse 100 ms in duration was transmitted into a smooth background F region plasma every 30 s. The 224 MHz radar measurements were made using the so-called coded long-pulse technique. This technique employs pseudo-random, binary phase-coded pulses to generate wideband spectra with good altitude resolution. Phase-coded pulses 1 ms in duration were transmitted within an interpulse period of 10 ms. The baud length of 1 m s yielded 1-MHz bandwidth spectra with 150 m altitude resolution. Simultaneous spectral measurements were made at the ion-line centered on the radar transmission frequency and at the HF-enhanced downshifted plasma line located at 224 MHz - f_HF, where f_HF is the frequency of the modifying HF transmissions.

Measurements of the radar backscatter spectra show that all key spectral features predicted by strong Langmuir turbulence (SLT) theory are present in the plasma and that their evolution is consistent with theoretical expectations. The results of the 1998 Tromsø campaign can be explained within the framework of the SLT work of DuBois et al. [J. Geophys. Res., 98, 17543-17567, 1993]. This theory yields a self-consistent description of the early time development of ion and Langmuir oscillations in the plasma above Tromsø under conditions of a very smooth ionospheric plasma. No other theory explains the early-time experimental results obtained at Arecibo, Puerto Rico and Tromsø, Norway in such a comprehensive and self-consistent manner. However, the theory of DuBois et al. is limited in that it does not address instability processes at work in an irregular plasma medium. Most HF-enhanced plasma line and ion line observations monitored in the past at Arecibo and Tromsø were made in the presence of HF-induced and/or natural irregularities. In this environment, the relative roles of SLT and parametric processes have yet to be determined.