Interpretation of HF radar ionospheric Doppler spectra by collective wave scattering theory

Abstract

According to the theory of collective wave scattering by a non-uniform plasma, the fluid motion spectrum can be either the Gaussian Doppler spectrum due to velocity dispersion, or a Lorentzian spectrum when the scattering wavelength becomes longer than the correlation length of velocity fluctuations. These two types of Doppler spectra are present in data from the SHERPA HF coherent radar, which detects 8 to 20-m wavelength density fluctuations in the high-latitude ionosphere. Most autocorrelation functions are best fitted to an exponential shape, corresponding to a Lorentzian Doppler spectrum. Nevertheless, the fit is sometimes better for a Gaussian shape. Each of the two spectral shapes is related to values of the local ionospheric parameters. We present preliminary measurements of the correlation time and correlation length of the waves, and concluding remarks on the significance of spectral widths for geophysical studies