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

Abstract

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. ,

First 100 ms of HF modification at Tromsø, Norway

“…The short answer is that the SLT models developed to date for ionospheric modification studies assume a smooth, horizontally stratified plasma, but experiments are frequently conducted in irregular ionospheric plasmas. The recent study of Mjølhus et al [2001] attempts to incorporate short‐scale irregularities into the instability process. However, much work remains to be performed in this area.…”

Geometric aspects of HF driven Langmuir turbulence in the ionosphere

“…The present authors hold the view that the success of "the first 10 ms" leads to a confidence in the theoretical understanding of the local process, and that explanations of the long time development of observations should be most of all sought in the development of the interface between local processes and radar observations, due to inhomogeneities. The paper (Mjølhus et al, 2001) attempts to contribute in this direction. In addition, modification of the electron distribution function may also have to be taken into account for a longer time scale understanding.…”

“…22 below, while the ion spectrum is about one-fifth of the latter. It is also interesting to compare with the results of Mjølhus et al (2001), where radar matching of numerically generated Langmuir spectra was promoted by refraction in plasma density ducts. Both virtual radar spectra of Fig.…”

“…Both virtual radar spectra of Fig. 8 of Mjølhus et al (2001) are about one-half of the corresponding spectra of Fig. 20.…”

Nonlinear trapping and self-guiding of magnetized Langmuir waves due to thermal plasma filamentation

“…[12][13][14][15] According to modern concepts, small-scale filamentation of the heated ionosphere occurs due to the nonuniform plasma heating by short-wavelength plasma oscillations. 12,[14][15][16][17][18] Plasma oscillations, in their turn, may be generated from the incident radio wave due to several processes, such as scattering of the incident wave on the initial plasma density fluctuations, 14 self-focusing of plasma waves, 16 parametric decay instability, etc. The crucial point of the model relates to the possibility of trapping of quasipotential waves inside a density irregularity.…”