Gradient Scattering Theory Demonstrates That VHF, Spread-F Radar Backscatter Reveals Streaming Instabilities

摘要

Field-aligned radar backscatter echoes have been obtained from the equatorial F region using frequencies from slightly above the plasma frequency to as high as 415 MHz. In this report we study the possibility that such echoes result from steep transitions from low- to high-density plasmas. We have studied four profile forms for this transition. The first, the only physically based profile, is assumed to be associated with the gyroradius distribution of the ions in the plasma. The second profile, the Epstein profile, is subject to exact reflectivity analysis and is used to provide guidance concerning the accuracy of the Born approximation used in our other calculations. The third profile, a half Gaussian wedded to a step function, a form used by Balsley and Farley, and a fourth profile, a raised cosine form, lead to very intense scattering, by hundreds of dBs, compared with that derived from our physically based gyroradius distribution. We believe we have shown that the intense scattering results computed by Balsley and Farley are due to mathematical discontinuities in the second derivatives of these last two profiles. As a result, we argue that the half-Gaussian profile is an inappropriate, unphysical model, and results based upon this model are probably unrealistic.