Abstract. The geometric aspects of HF-generated Langmuir turbulence in the ionosphere and its detection by radars are theoretically discussed in a broad approach, including local modelling (damped and driven Zakharov system), basic parametric instabilities, polarization and strength of the driving electric field, and radar configurations. Selected examples of numerical results from the local model are presented and discussed in relation to recent experiments, with emphasis on recent experiments at the EISCAT facilities. Anisotropic aspects of the cavitation process in the magnetized plasma are exhibited. Basic processes of cascades and cavitation are by now well identified in these experiments, but a few problems of the detailed agreement between theory and experiments are pointed out.

Abstract. In experiments where ground-based VHF/UHF radars are used to diagnose Langmuir turbulence driven by powerful HF waves transmitted from the ground into the ionosphere, there has been a severe observability aspect: The excited Langmuir wave vector spectrum must contain a component satisfying the actual radar Bragg condition. In order to explain earlier observations, in particular those made by the Arecibo facility, it was proposed long ago that the actual wave number spectra were widened by refraction in small-scale irregularities, or ducts. This mechanism of improved observability is discussed in a broad approach in this paper. The main contribution is a mathematical derivation of the theory of parametric decay instability in the presence of magnetic-field-aligned ducts. This implies that the decay is into discrete duct modes. The threshold is slightly higher and the growth rate is slightly lower than for a corresponding homogeneous medium. Moreover, this tendency is more pronounced for the higher duct modes, which are those that may satisfy conditions for observability. The mathematical derivation is based on the driven and damped Zakharov model with ducts included. The model is run numerically, confirming the approximate decay instability theory and demonstrating improved observability. The numerics also demonstrate higher Langmuir energy density inside the ducts than outside. ,

Langmuir turbulence during ionospheric heating experiments is studied by a model which treats the low‐frequency ion dynamics from a linearized kinetic description, where the usual high frequency Zakharov equation describing the electron dynamics is retained. The main objective is to compare the predictions from this model with results previously obtained from quasi‐fluid Zakharov models. Our findings are that the predictions from the two types of models are in good qualitative agreement. At high values of the frequency mismatch and moderate values of the driving electric field, the saturated dynamics is cascade‐dominated, while at lower‐frequency mismatch, that is, nearer to the reflection height, or at higher drive, the dynamics is of the cavitation type. In cascade dominated cases the perturbation of the ion velocity distribution is mainly around the ion acoustic velocity, while in cavitation‐dominated cases the whole velocity range is perturbed.