Observation of the high latitude ionosphere by HF radars: Interpretation in terms of collective wave scattering and characterization of turbulence

HF coherent radars are sensitive to electron density fluctuations of the ionospheric plasma that have a wavelength of half the radar wavelength. The SHERPA HF radar transmits sequences of seven pulses from which a 17 lag complex autocorrelation function of the returned signal is obtained. A study of the autocorrelation function gives access to the mean Doppler velocity in the scattering volume and to the spectral width. A detailed study has shown that the variation with time of the power of the autocorrelation function can be approximated by a Gaussian or Lorentzian function or, more usually, by an intermediate form of these two profiles, beginning with a Gaussian and finishing with a Lorentzian. Here, we propose an analytic form for the autocorrelation function that satisfies these tendencies. It is based on a Lagrangian description of the diffusion centre's motion. From this analysis, the turbulent diffusion coefficient, the correlation length and the correlation time associated with the turbulence can be directly estimated by fitting the data with this analytic expression. An initial study, over a limited data sample obtained with the SHERPA HF radar, is presented. In particular, the observed turbulent diffusion coefficient is two orders of magnitude greater than the classical coefficient. (C) 1996 Elsevier Science Ltd.