No abstract

Experimental results, obtained with the Heating facility and the EISCAT UHF and VHF radars at Tromsø, Norway, have been used to test existing Langmuir turbulence theories and to provide further observational facts to be taken into account in a new theoretical formulation of Langmuir turbulence. It is found that none of the available theories is able to describe the diversity of observational facts. The dominance of propagating Langmuir and ion acoustic waves in the observed spectra, and the fact that these waves closely follow their linear dispersion relations, lend support to a weak turbulence description. On the other hand, the observation that the number of cascade steps is limited to two, independent of input power, strongly contradicts such an approach. Unmistakable indications for the occurrence of caviton collapse are not seen in the experimental results at Tromsø. The low‐frequency turbulence is found to consist of subsonic and sonic contributions, showing widely independent properties. The sonic turbulence is seen to evolve from the natural, thermally excited, ion acoustic background.

No abstract

Abstract. Measurements of plasma and ion lines induced during HF ionospheric interaction experiments have been made with the European Incoherent Scatter (EISCAT) facility at TromsO with sufficiently high-altitude resolution to compare with theories of Langmuir turbulence. Recent Langmuir turbulence models predict a change from broad structureless spectra to line or cascade spectra within a few hundred meters for VHF (224 MHz) observations assuming typical ionospheric density gradients. In a campaign in May 1994 we found VHF spectra that were grouped into two regions separated in altitude by -2 km, with broad, unstructured plasma line spectra in the upper region and cascade type spectra in the lower region. The ion line channels showed detectable spectra mainly in the upper altitude region, which corresponds to that which had the broad plasma lines. The background ionospheric density profile showed an unusually low plasma density gradient near the HF reflection heights, thus allowing the two regions, which are normally so close together that one only sees a transition from one type of spectra to the other, to be clearly separated in height. Thus, in the high-latitude ionosphere there can, at times, be a simultaneous existence in spatially separate regions of cavitation (often referred to as strong turbulence) and cascading (normally associated with saturated parametric decay) as predicted by some simulations. Another new feature is a height variation in the plasma line cascades with the highest-order cascades strongest at the lowest heights, in accordance with expectations based on the parametric decay instability. Using the pulse-to-pulse technique, a continuous transition 7429

No abstract