Significance of short-scale irregularities for radar diagnostics of HF-driven Langmuir turbulence in the ionosphere

In experiments where ground-based VHF/UHF radars are used to diagnose Langm uir 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 actua l radar Bragg condition. In order to explain earlier observations, in parti cular those made by the Arecibo facility, it was proposed long ago that the actual wave number spectra were widened by refraction in small-scale irreg ularities, 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 slight ly lower than for a corresponding homogeneous medium. Moreover, this tenden cy is more pronounced for the higher duct modes, which ate those that may s atisfy conditions for observability. The mathematical derivation is based o n the driven and damped Zakharov model with ducts included. The model is ru n numerically, confirming the approximate decay instability theory and demo nstrating improved observability. The numerics also demonstrate higher Lang muir energy density inside the ducts than outside.