Ray tracing synthesis of HF radar signatures from Gaussian plasma cloud

Abstract

HF radar signatures have been synthesized from a time-dependent Gaussian ion cloud using a three-dimensional ray-tracing progam that can take into account both electron collisions and the earth's magnetic field. The ion cloud model is based on the theory of ambipolar diffusion and the model parameters are based upon the observations made during a barium cloud test conducted at White Sands, New Mexico. Ray-tracing results have been generated for both ordinary (O) and extraordinary(X) modes as well as for the "no-field" case. The results, presented in the form of a time-history of the Doppler shift and the scattering cross section, are found to be quite similar for all three cases. Relative to the O mode, the X-mode variations lag in time and obtain a maximum cross section which is greater only by about 2 db. The variations for the no-field case fall in between that of the O and X, but closer to the O mode. The results show that the cloud is extremely defocussing with an estimated defocussing factor (relative to free space) for the X mode increasing from 7.6 × 10³ to 1.85 × 10⁷ over the interval 5 to 190 sec of cloud's life. The ray-tracing signatures are compared to those synthesized by means of a hard expanding ellipsoid method with a view to evaluating the importance of underlying ionization. The effects are found to be such as to introduce a significant time lag in Doppler history and to cause a cross section reduction by a factor that builds up with time from 5 db to more than 30 db. A comparison drawn between the observed and the ray-tracing signatures leads to the conclusion that good agreement between the two can be expected only when the clouds experience either slow or no deformation and striation but not otherwise.