Radio frequency and scattering angle dependence of ionospheric scatter propagation at VHF

The weak and fluctuating radio signals observed at distances of 1,500 km on VHF are attributed to scattering from E-region turbulence. It is noted that propagation constants k = 4π/λ sin (θ/2), corresponding to the experimental frequencies (28 to 108 Mc), just straddle the viscosity cutoff wave-number ks = (2 meters)−1 of the region, thereby giving a qualitative explanation for the curious dichotomy found in the experimental data. The two competitive turbulence theories are then developed in detail near the viscosity transition range. It is found that pressure fluctuations of the ionosphere's neutral gases induce a spectrum of dielectric variations which reproduce the frequency and scattering angle dependence of the experimental results quite well. A theory of turbulent mixing of ionization gradients is then developed along the lines of Heisenberg's original treatment of the velocity field. This process predicts a frequency variation of power levels which is also satisfactory. It is concluded that more precise, simultaneous measurements will be required to choose between the two theories on this basis.