Equatorial spread-F and neutral atmospheric turbulence : a review and a comparative anatomy

A number of satellite and rocket plasma density spectra obtained during equatorial spread-F conditions are presented and discussed in the light of similar measurements in the neutral atmosphere. We discuss this comparison in some detail and find both distinct similarities and subtle differences. The horizontal spectral measurements show a peak at an outer scale quite similar to the scale of the undulations caused by gravity wave interactions with the ionosphere. This feature is similar to a buoyancy subrange but it is easy to show that the amplitudes of the plasma fluctuations are too large to be directly driven by the neutral atmosphere. At intermediate scales the plasma fluctuations have a one-dimensional horizontal spectrum with a power law well described by a ($\text{−5}\text{3}$) slope. Once again it can be shown that the neutral fluid cannot be similarly structured at 400 km altitude due to the high viscosity coefficient. A plasma cascade process seems to be operating but it is not at all clear how the spectrum is formed. Furthermore, vertical power spectra seem to run the gamut in spectral form with slopes (n) varying in the range from −1 to −3. So the horizontal spectra are near universal in form, while the vertical spectra are quite variable.