Space and time distribution of HF excited Langmuir turbulence in the ionosphere: Comparison of theory and experiment

Abstract

The predictions of strong Langmuir turbulence (SLT) theory are compared with radar observations of HF induced turbulence at Arecibo and Tromsø. The altitude distribution of turbulence observed in the cold start experiments of Fejer et al. (1991) imply that the ionospheric electron density profile is modified by the induced turbulence. The preconditioned observations at Arecibo and the Tromsø observations also appear to require a “disturbed” profile with several percent density fluctuations. With such density modifications postulated we conclude that SLT theory is in, at least, qualitative agreement with a large body of observations. Specifically SLT theory predicts, as part of a unified theory, and in distinction to the weak turbulence approximation, at least four unique physical signatures which can be compared to observations: (1.) A caviton continuum plus free mode peak in the plasma line power spectrum near reflection altitude for Arecibo conditions. (2.) A truncated decay‐cascade spectrum at lower altitudes (or densities). (3.) A continuous spectrum underlying the decay‐cascade spectrum. (4.) A zero frequency feature in the ion line power spectrum directly related to caviton dynamics. We find that there is sufficient ponderomotive pressure due to the Airy‐layered, induced Langmuir turbulence, to modify the electron density profile in a manner consistent with the time behavior of unpreconditioned Arecibo observations.